git-filter-repo.txt: add documentation of --no-ff option

Commit 5e04dff097 (filter-repo: add new --no-ff option, 2020-01-01)
added support for a --no-ff option, but only added documentation in the
built-in output, not in the intended-to-be-more-complete manual.  Add
documentation to the manual for this option.

Signed-off-by: Elijah Newren <newren@gmail.com>

Documentation/git-filter-repo.txt

@@ -222,6 +222,12 @@ Parent rewriting
 	('auto') is to only prune empty merge commits which become
 	degenerate (not which started as such).
 
+--no-ff::
+	Even if the first parent is or becomes an ancestor of another
+	parent, do not prune it.  This modifies how --prune-degenerate
+	behaves, and may be useful in projects who always use merge
+	--no-ff.
+
 Generic callback code snippets
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~