config.txt: clarify core.checkStat

The description of this key does not really tell what the 'minimal'
mode checks and does not check.  The description for the 'default'
mode is not much better and just says 'all fields', which is unclear
and is not even correct (e.g. we do not look at 'atime').

Spell out what are and what are not checked under the 'minimal' mode
relative to the 'default' mode to help those who want to decide if
they want to use the 'minimal' mode, also taking information about
this mode from the commit message of c08e4d5b5c (Enable minimal stat
checking - 2013-01-22).

Helped-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -449,10 +449,20 @@ core.untrackedCache::
 	See linkgit:git-update-index[1]. `keep` by default.
 
 core.checkStat::
-	Determines which stat fields to match between the index
-	and work tree. The user can set this to 'default' or
-	'minimal'. Default (or explicitly 'default'), is to check
-	all fields, including the sub-second part of mtime and ctime.
+	When missing or is set to `default`, many fields in the stat
+	structure are checked to detect if a file has been modified
+	since Git looked at it.  When this configuration variable is
+	set to `minimal`, sub-second part of mtime and ctime, the
+	uid and gid of the owner of the file, the inode number (and
+	the device number, if Git was compiled to use it), are
+	excluded from the check among these fields, leaving only the
+	whole-second part of mtime (and ctime, if `core.trustCtime`
+	is set) and the filesize to be checked.
++
+There are implementations of Git that do not leave usable values in
+some fields (e.g. JGit); by excluding these fields from the
+comparison, the `minimal` mode may help interoperability when the
+same repository is used by these other systems at the same time.
 
 core.quotePath::
 	Commands that output paths (e.g. 'ls-files', 'diff'), will