gh-90120: Specify UTF-8 encoding in PyModule_AddString{Constant,Macro} docs

[dan-wilton]

This is my first commit(s) and PR to cpython, thought I'd pick up some docs updates first 📝😄

Issue: #90120

• Issue: Clarify that PyModule_AddString{Constant,Macro} use utf-8 #90120

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📚 Documentation preview 📚: https://cpython-previews--131597.org.readthedocs.build/

[ghost]

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[ZeroIntensity]

value is not a variable we know about here. It makes sense for PyModule_AddStringConstant because there's a value parameter.

Instead, let's do something like this:

Suggested change

	*STRINGLIB_TYPE_NAME* to *module*. The string *value* must be
	``NULL``-terminated and UTF-8 encoded.
	*STRINGLIB_TYPE_NAME* to *module*. The contents of the macro must be
	``NULL``-terminated and UTF-8 encoded.

I think it would also make more sense to move that sentence to before "For example".

Taking a second look