Documentation/git-gc: improve description of --auto

Documentation/git-gc: improve description of --auto

This patch tries to make the description of --auto a little
more clear for new users, especially those referred by the
"git-gc --auto" notification message.

It also cleans up some grammatical errors and typos in the
original description, as well as rewording for clarity.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Shawn O. Pearce <redacted>