Documentation/git-rebase.txt: document when --fork-point is auto-enabled

Running "git rebase" without giving a specific commit with respect
to which the operation is done enables --fork-point mode, while
telling the command to rebase with respect to a specific commit,
i.e. "git rebase <upstream>" does not.

This was not mentioned in the DESCRIPTION section of the manual
page, even though the case of omitted <upstream> was otherwise
discussed.  That in turn made actual behavior of vanilla "git
rebase" hardly discoverable.

While we are at it, clarify the --fork-point description itself as
well.

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>