runtime(doc): document gitcommit_summary_length

related: #19905

Signed-off-by: Christian Brabandt <[email protected]>

Author: chrisbra

runtime/doc/filetype.txt

@@ -1,4 +1,4 @@
-*filetype.txt*	For Vim version 9.2.  Last change: 2026 Mar 24
+*filetype.txt*	For Vim version 9.2.  Last change: 2026 Apr 07
 
 
 		  VIM REFERENCE MANUAL	  by Bram Moolenaar
@@ -652,6 +652,13 @@ One command, :DiffGitCached, is provided to show a diff of the current commit
 in the preview window.  It is equivalent to calling "git diff --cached" plus
 any arguments given to the command.
 
+
+The length of the first line of the commit message used for
+syntax highlighting can be configured via `g:gitcommit_summary_length`.
+The default is 50.  Example: >
+
+        let g:gitcommit_summary_length = 70
+
 GIT REBASE						*ft-gitrebase-plugin*
 
 The gitrebase filetype defines the following buffer-local commands, to help