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>