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

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>