Clarify that _foo and _bar are only type wildcards with -XNamedWildCards

Summary

The beginning of the section "Partial Type Signatures" in glasgow_exts.rst in the users' guide reads

A partial type signature is a type signature containing special placeholders written with a leading underscore (e.g., "_", "_foo", "_bar") called wildcards.

That "foo" and "bar" are, even if with -XPartialTypeSignatures, by default type variables rather than wildcards, only becomes clear if the user reads about it several paragraphs and subsection headers further down, in "Named Wildcards".

This should be made more obvious in the beginning of the section.

Proposed improvements or changes

Change the text to something like

A partial type signature is a type signature containing special placeholders called wildcards. A wildcard is written as an underscore ("_"), or, if NamedWildCards is enabled, any identifier with a leading underscore (e.g., "_", "_foo", "_bar").