Allow escaping values defined in "docstring statements" such as tags

[clebergnu]

Time and time again we expand the definition of the DOCSTRING_DIRECTIVE_RE_RAW in an attempt to allow for more descriptive tags.

This time there's an argument in favor of allowing commas to be used as part of tag values.

To cover this and future use cases, let's evaluate (and if possible) come up with a general and universal way of escaping characters in the docstring statements.