Clean up the macros chapter

[sanxiyn]

It would be nice to use consistent terminology for macros.

For example, in $a:b, Macros Guide calls b a "fragment specifier", Reference Manual calls it a "designator". Both are inscrutable, and looking at search engine hits "fragment specifier" seems to be a Rust coinage.

Dylan calls it "constraint", which seems reasonable to me. See a section on Dylan Macro System documentation.

[steveklabnik]

/cc @aturon

[aturon]

@steveklabnik Do you plan to write docs for writing macros? Our current docs there are very sparse. If/when you do that, I'd be glad to discuss terminology, though I'm a total novice.

[steveklabnik]

Yes, eventually, the macro guide is being completely redone.

[steveklabnik]

/cc @kmcallister , what's your opinion here?

[kmcallister]

The macros chapter could use a lot of work. I think we should find more exciting examples for everything before "Hygiene" and then rewrite the explanation around those. I can take a shot at this before 1.0-final.

[kmcallister]

In progress as #22393.

[kmcallister]

Landed! ^_^