merge-base --fork-point doc: clarify the example and failure modes

The illustrated history used to explain the `--fork-point` mode
named three keypoint commits B3, B2 and B1 from the oldest to the
newest, which was hard to read.  Relabel them to B0, B1, B2.  Also
illustrate the history after the rebase using the `--fork-point`
facility was made.

The text already mentions use of reflog, but the description is not
clear what benefit we are trying to gain by using reflog.  Clarify
that it is to find the commits that were known to be at the tip of
the remote-tracking branch.  This in turn necessitates users to know
the ramifications of the underlying assumptions, namely, expiry of
reflog entries will make it impossible to determine which commits
were at the tip of the remote-tracking branches and we fail when in
doubt (instead of giving a random and incorrect result without even
warning).  Another limitation is that it won't be useful if you did
not fork from the tip of a remote-tracking branch but from in the
middle.

Describe them.

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

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