Improve documentation for lsp-execute-code-action (#4097)

`lsp-execute-code-action` is a user-facing function that can be
executed e.g. when clangd says "fix available". But as it stands, the
function description is too abstract, so unless you know details
around lsp-protocol `codeAction`, you won't figure out what it's
for. Fix that by clarifying what a user can use it for.

lsp-mode.el

@@ -5873,7 +5873,9 @@ It will filter by KIND if non nil."
          (lsp-send-execute-command command arguments?))))))
 
 (lsp-defun lsp-execute-code-action ((action &as &CodeAction :command? :edit?))
-  "Execute code action ACTION.
+  "Execute code action ACTION. For example, when text under the
+caret has a suggestion to apply a fix from an lsp-server, calling
+this function will do so.
 If ACTION is not set it will be selected from `lsp-code-actions-at-point'.
 Request codeAction/resolve for more info if server supports."
   (interactive (list (lsp--select-action (lsp-code-actions-at-point))))