Document -B<n>[/<m>], -M<n> and -C<n> variants of -B, -M and -C

Document -B<n>[/<m>], -M<n> and -C<n> variants of -B, -M and -C

These options take an optional argument, but this optional argument was
not documented.

Original patch by Matthieu Moy, but documentation for -B mostly copied
from the explanations of Junio C Hamano.

While we're there, fix a typo in a comment in diffcore.h.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>