git-gc --auto: add documentation.

This documents the auto-packing of loose objects performed by
git-gc --auto.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -439,6 +439,13 @@ gc.aggressiveWindow::
 	algorithm used by 'git gc --aggressive'.  This defaults
 	to 10.
 
+gc.auto::
+	When there are approximately more than this many loose
+	objects in the repository, `git gc --auto` will pack them.
+	Some Porcelain commands use this command to perform a
+	light-weight garbage collection from time to time.  Setting
+	this to 0 disables it.
+
 gc.packrefs::
 	`git gc` does not run `git pack-refs` in a bare repository by
 	default so that older dumb-transport clients can still fetch

Documentation/git-gc.txt

@@ -8,7 +8,7 @@ git-gc - Cleanup unnecessary files and optimize the local repository
 
 SYNOPSIS
 --------
-'git-gc' [--prune] [--aggressive]
+'git-gc' [--prune] [--aggressive] [--auto]
 
 DESCRIPTION
 -----------
@@ -43,6 +43,15 @@ OPTIONS
 	persistent, so this option only needs to be used occasionally; every
 	few hundred changesets or so.
 
+--auto::
+	With this option, `git gc` checks if there are too many
+	loose objects in the repository and runs
+	gitlink:git-repack[1] with `-d -l` option to pack them.
+	The threshold is set with `gc.auto` configuration
+	variable, and can be disabled by setting it to 0.  Some
+	Porcelain commands use this after they perform operation
+	that could create many loose objects automatically.
+
 Configuration
 -------------