whatschanged: list it in BreakingChanges document

whatschanged: list it in BreakingChanges document

This can be squashed into the previous step.  That is how our "git
pack-redundant" conversion did.

Theoretically, however, those who want to gauge the need to keep the
command by exposing their users to patches before this one may want
to wait until their experiment finishes before they formally say
"this will go away".

This change is made into a separate patch from the previous step
precisely to help those folks.

While at it, update the documentation page to use the new [synopsis]
facility to mark-up the SYNOPSIS part.

Helped-by: Elijah Newren <newren@gmail.com>
[en: typofix]
Signed-off-by: Junio C Hamano <gitster@pobox.com>