Fine-tune gitignore

The pattern "*.1" is intended to match manpage files built in the "doc"
directory, but it was also matching distribution directories like
"yash-2.56.1". To avoid this mismatch, this commit adds the explicit
prefix to the pattern.

Author: magicant

.gitignore

@@ -5,13 +5,6 @@ TAGS
 tags
 cscope.out
 
-*.1
-*.html
-*.xhtml
-*.xml
-index.txt
-yash.txt
-
 /yash
 /makesignum
 /signum.h
@@ -22,6 +15,13 @@ yash.txt
 
 /builtins/builtins.a
 
+/doc/**/*.1
+/doc/**/*.html
+/doc/**/*.xhtml
+/doc/**/*.xml
+/doc/**/index.txt
+/doc/**/yash.txt
+
 /lineedit/lineedit.a
 /lineedit/commands.in