Documentation/git-rebase.txt: -f forces a rebase that would otherwise be a no-op

"Current branch is a descendant of the commit you are rebasing onto"
does not necessarily mean "rebase" requires "--force".  For a plain
vanilla "history flattening" rebase, the rebase can be done without
forcing if there is a merge between the tip of the branch being
rebased and the commit you are rebasing onto, even if the tip is
descendant of the other.

[jc: reworded both the text and the log description]

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

Documentation/git-rebase.txt

@@ -316,11 +316,8 @@ which makes little sense.
 
 -f::
 --force-rebase::
-	Force the rebase even if the current branch is a descendant
-	of the commit you are rebasing onto.  Normally non-interactive rebase will
-	exit with the message "Current branch is up to date" in such a
-	situation.
-	Incompatible with the --interactive option.
+	Force a rebase even if the current branch is up-to-date and
+	the command without `--force` would return without doing anything.
 +
 You may find this (or --no-ff with an interactive rebase) helpful after
 reverting a topic branch merge, as this option recreates the topic branch with