git-rebase: Update --whitespace documentation

git-rebase: Update --whitespace documentation

The parameters accepted by the --whitespace option of "git apply" have
changed over time, and the documentation for "git rebase" was out of
sync.  Remove the specific parameter list from the "git rebase"
documentation and simply point to the "git apply" documentation for
details, as is already done in the "git am" documentation.

Signed-off-by: Todd Zullinger <tmz@pobox.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>