rerere: Document 'rerere remaining'

This adds the 'remaining' command to the documentation of
'git rerere'. This command was added in ac49f5ca (Feb 16 2011;
Martin von Zweigbergk <martin.von.zweigbergk@gmail.com>) but
it was never documented.

Touch up the other rerere commands to reduce noise.

First noticed by Vincent van Ravesteijn.

Signed-off-by: Phil Hord <phil.hord@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>