git-rebase.txt: address confusion between --no-ff vs --force-rebase

git-rebase.txt: address confusion between --no-ff vs --force-rebase

rebase was taught the --force-rebase option in commit b2f82e05de ("Teach
rebase to rebase even if upstream is up to date", 2009-02-13).  This flag
worked for the am and merge backends, but wasn't a valid option for the
interactive backend.

rebase was taught the --no-ff option for interactive rebases in commit
b499549401cb ("Teach rebase the --no-ff option.", 2010-03-24), to do the
exact same thing as --force-rebase does for non-interactive rebases.  This
commit explicitly documented the fact that --force-rebase was incompatible
with --interactive, though it made --no-ff a synonym for --force-rebase
for non-interactive rebases.  The choice of a new option was based on the
fact that "force rebase" didn't sound like an appropriate term for the
interactive machinery.

In commit 6bb4e485cff8 ("rebase: align variable names", 2011-02-06), the
separate parsing of command line options in the different rebase scripts
was removed, and whether on accident or because the author noticed that
these options did the same thing, the options became synonyms and both
were accepted by all three rebase types.

In commit 2d26d533a012 ("Documentation/git-rebase.txt: -f forces a rebase
that would otherwise be a no-op", 2014-08-12), which reworded the
description of the --force-rebase option, the (no-longer correct) sentence
stating that --force-rebase was incompatible with --interactive was
finally removed.

Finally, as explained at
https://public-inbox.org/git/98279912-0f52-969d-44a6-22242039387f@xiplink.com

    In the original discussion around this option [1], at one point I
    proposed teaching rebase--interactive to respect --force-rebase
    instead of adding a new option [2].  Ultimately --no-ff was chosen as
    the better user interface design [3], because an interactive rebase
    can't be "forced" to run.

We have accepted both --no-ff and --force-rebase as full synonyms for all
three rebase types for over seven years.  Documenting them differently
and in ways that suggest they might not be quite synonyms simply leads to
confusion.  Adjust the documentation to match reality.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>