Join "optional" and "extra_info" rules in grammar

[lagru]

Since the "optional" statement isn't used in any way right now, enforcing a specific layout for what comes after the "doctype" serves no useful purpose. I don't intend docstub to be linter so I don't think it's likely that we will ever enforce annotating that a parameter is "optional" in the docstring – while it's nice to do so, it's reasonably easy to see it from the signature.

This also makes it easy to add info like requested in #65. It's not used in any way but it's possible to include in docstrings. Docstub shouldn't make it harder writing rich docstrings if possible.

Update the docs accordingly. Also try to be more precise about differentiating "doctypes" from valid Python types.

Release note

For maintainers and optionally contributors, please refer to changelist's README on how to document this PR for the release notes.

Relax grammar for specifying optional and additional information
after type descriptions in docstrings. You can now specify as much
optional information after the first top-level comma as you want
and in arbitrary order, e.g., `some_param: int, in range (0, 1), optional`.
This optional information is ignored by docstub.
{label="enhancement"}