feat(defaults): session data in $XDG_STATE_HOME neovim#15583

See: https://gitlab.freedesktop.org/xdg/xdg-specs/-/commit/4f2884e16db35f2962d9b64312917c81be5cb54b

- Move session persistent data to $XDG_STATE_HOME Change 'directory',
  'backupdir', 'undodir', 'viewdir' and 'shadafile' default location to
  $XDG_STATE_HOME/nvim.
- Move logs to $XDG_STATE_HOME, too.
- Add stdpath('log') support.

Fixes: neovim#14805

Author: etircopyh

man/nvim.1

@@ -188,7 +188,7 @@ loading plugins is also skipped.
 Use
 .Ar shada
 instead of the default
-.Pa ~/.local/share/nvim/shada/main.shada .
+.Pa ~/.local/state/nvim/shada/main.shada .
 If
 .Ar shada
 is
@@ -326,7 +326,7 @@ Print version information and exit.
 .Sh ENVIRONMENT
 .Bl -tag -width Fl
 .It Ev NVIM_LOG_FILE
-Low-level log file, usually found at ~/.cache/nvim/log.
+Low-level log file, usually found at ~/.local/state/nvim/log.
 :help $NVIM_LOG_FILE
 .It Ev VIM
 Used to locate user files, such as init.vim.
@@ -340,12 +340,20 @@ Path to the user-local configuration directory, see
 Defaults to
 .Pa ~/.config .
 :help xdg
-.It Ev XDG_DATA_HOME
+.It Ev XDG_STATE_HOME
 Like
 .Ev XDG_CONFIG_HOME ,
 but used to store data not generally edited by the user,
 namely swap, backup, and ShaDa files.
 Defaults to
+.Pa ~/.local/state .
+:help xdg
+.It Ev XDG_DATA_HOME
+Like
+.Ev XDG_CONFIG_HOME ,
+but used to store data not generally edited by the user,
+things like runtime files.
+Defaults to
 .Pa ~/.local/share .
 :help xdg
 .It Ev VIMINIT

runtime/doc/builtin.txt

@@ -7465,14 +7465,17 @@ stdpath({what})					*stdpath()* *E6100*
 		directories.
 
 		{what}       Type    Description ~
-		cache        String  Cache directory. Arbitrary temporary
+		cache        String  Cache directory: arbitrary temporary
 		                     storage for plugins, etc.
-		config       String  User configuration directory. The
-		                     |init.vim| is stored here.
-		config_dirs  List    Additional configuration directories.
+		config       String  User configuration directory. |init.vim|
+		                     is stored here.
+		config_dirs  List    Other configuration directories.
 		data         String  User data directory. The |shada-file|
 		                     is stored here.
-		data_dirs    List    Additional data directories.
+		data_dirs    List    Other data directories.
+		log          String  Logs directory (for use by plugins too).
+		state        String  Session state directory: storage for file
+				     drafts, undo history, shada, etc.
 
 		Example: >
 			:echo stdpath("config")

runtime/doc/options.txt

@@ -841,7 +841,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	again not rename the file.
 
 						*'backupdir'* *'bdir'*
-'backupdir' 'bdir'	string	(default ".,$XDG_DATA_HOME/nvim/backup//")
+'backupdir' 'bdir'	string	(default ".,$XDG_STATE_HOME/nvim/backup//")
 			global
 	List of directories for the backup file, separated with commas.
 	- The backup file will be created in the first directory in the list
@@ -2063,7 +2063,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	{char2}.  See |digraphs|.
 
 						*'directory'* *'dir'*
-'directory' 'dir'	string	(default "$XDG_DATA_HOME/nvim/swap//")
+'directory' 'dir'	string	(default "$XDG_STATE_HOME/nvim/swap//")
 			global
 	List of directory names for the swap file, separated with commas.
 
@@ -3502,7 +3502,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	option.  For '@' only characters up to 255 are used.
 	Careful: If you change this option, it might break expanding
 	environment variables.  E.g., when '/' is included and Vim tries to
-	expand "$HOME/.local/share/nvim/shada/main.shada".  Maybe you should 
+	expand "$HOME/.local/state/nvim/shada/main.shada".  Maybe you should
 	change 'iskeyword' instead.
 
 						*'iskeyword'* *'isk'*
@@ -4942,9 +4942,12 @@ A jump table for the options with a short description can be found at |Q_op|.
 	   but are not part of the Nvim distribution. XDG_DATA_DIRS defaults
 	   to /usr/local/share/:/usr/share/, so system administrators are
 	   expected to install site plugins to /usr/share/nvim/site.
-	5. $VIMRUNTIME, for files distributed with Neovim.
+	5. Applications state home directory, for files that contain your
+	   session state (eg. backupdir, viewdir, undodir, etc).
+	   Given by `stdpath("state")`.  |$XDG_STATE_HOME|
+	6. $VIMRUNTIME, for files distributed with Neovim.
 							*after-directory*
-	6, 7, 8, 9. In after/ subdirectories of 1, 2, 3 and 4, with reverse 
+	7, 8, 9, 10. In after/ subdirectories of 1, 2, 3 and 4, with reverse
 	   ordering.  This is for preferences to overrule or add to the 
 	   distributed defaults or system-wide settings (rarely needed).
 
@@ -6623,7 +6626,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 'ttyfast' 'tf'		Removed. |vim-differences|
 
 						*'undodir'* *'udir'* *E5003*
-'undodir' 'udir'	string	(default "$XDG_DATA_HOME/nvim/undo//")
+'undodir' 'udir'	string	(default "$XDG_STATE_HOME/nvim/undo//")
 			global
 	List of directory names for undo files, separated with commas.
 	See 'backupdir' for details of the format.
@@ -6786,7 +6789,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	displayed when 'verbosefile' is set.
 
 						*'viewdir'* *'vdir'*
-'viewdir' 'vdir'	string	(default: "$XDG_DATA_HOME/nvim/view//")
+'viewdir' 'vdir'	string	(default: "$XDG_STATE_HOME/nvim/view//")
 			global
 	Name of the directory where to store files for |:mkview|.
 	This option cannot be set from a |modeline| or in the |sandbox|, for