filter-repo: clarify usage of --use-base-name

fast-export/fast-import only work with filenames (using full path from
the root of the repository); thus that's all that filter-repo works
with.  Full pathnames implicitly include all leading directories as part
of the pathname, which is what allows us to match against directories.
However, it obviously means --use-base-name can't be used to match paths
against directories.

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

Documentation/git-filter-repo.txt

@@ -106,7 +106,8 @@ Filtering based on paths (see also --filename-callback)
 
 --use-base-name::
 	Match on file base name instead of full path from the top of
-	the repo.  Incompatible with --path-rename.
+	the repo.  Incompatible with --path-rename, and incompatible
+	with matching against directory names.
 
 Renaming based on paths (see also --filename-callback)
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

git-filter-repo

@@ -1777,7 +1777,8 @@ EXAMPLES
                "paths"))
     path.add_argument('--use-base-name', action='store_true',
         help=_("Match on file base name instead of full path from the top "
-               "of the repo.  Incompatible with --path-rename."))
+               "of the repo.  Incompatible with --path-rename, and "
+               "incompatible with matching against directory names."))
 
     rename = parser.add_argument_group(title=_("Renaming based on paths "
                                              "(see also --filename-callback)"))