man: drop rhs parts in snippets in directive index

-       --exit-if-exists=file
+       --exit-if-exists=
etc.

make-directive-index.py

@@ -185,7 +185,10 @@ def _extract_directives(directive_groups, formatting, page):
                 stor[text].append((pagename, section))
                 if text not in formatting:
                     # use element as formatted display
-                    name.tail = ''
+                    if name.text[-1] in '= ':
+                        name.clear()
+                    else:
+                        name.tail = ''
                     name.text = text
                     formatting[text] = name