GeezerVison

Change pydoc_data to show the "input" of grammar snippet
directives, which is formatted the way we want.

See also this Sphinx issue: sphinx-doc/sphinx#13302
And: sphinx-doc/sphinx#4454

A known issue is cross-grammar links, like those in
the string format specification mini-language, which render
as reST source:

    element_index:     `~python-grammar:digit`+ | `index_string`

I don't see a good way to fix this before Sphinx is refactored.

This is usually done at release time.