docs: "Return (multiple)" heading

Problem:
Lua functions that return multiple results are declared by using
multiple `@return` docstring directives. But the generated docs don't
make it obvious what this represents.

Solution:
- Generate a "Return (multiple)" heading for multiple-value functions.
- Fix `@note` directives randomly placed after `@return`.

scripts/gen_vimdoc.py

@@ -745,7 +745,7 @@ def fmt_node_as_vimhelp(parent, width=text_width - indentation, indent='',
             chunks.append('\nParameters: ~')
             chunks.append(fmt_param_doc(para['params']))
         if len(para['return']) > 0:
-            chunks.append('\nReturn: ~')
+            chunks.append('\nReturn (multiple): ~' if len(para['return']) > 1 else '\nReturn: ~')
             for s in para['return']:
                 chunks.append(s)
         if len(para['seealso']) > 0: