Documentation/git-gc: explain --auto in description

Now that git-gc --auto tells the user to look at the man
page, it makes sense to mention the auto behavior near the
top (since this is likely to be most users' first exposure
to git-gc).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Documentation/git-gc.txt

@@ -19,7 +19,8 @@ created from prior invocations of gitlink:git-add[1].
 
 Users are encouraged to run this task on a regular basis within
 each repository to maintain good disk space utilization and good
-operating performance.
+operating performance. Some git commands may automatically run
+`git-gc`; see the `--auto` flag below for details.
 
 OPTIONS
 -------