Documentation: point git-prune users to git-gc

Documentation: point git-prune users to git-gc

Most users should be using git-gc instead of directly
calling prune. For those who really do want more information
on pruning, let's point them at git-fsck, which goes into
slightly more detail on reachability.

And since we're pointing users there, let's make sure
reflogs are mentioned in git-fsck(1).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>