Rollup merge of #61606 - petrochenkov:legclean, r=pnkfelix

Remove some legacy proc macro flavors

Namely
- `IdentTT` (`foo! ident { ... }`). Can be replaced with `foo! { ident ... }` or something similar.
- `MultiDecorator`. Can be replaced by `MultiModifier` (aka `LegacyAttr` after renaming).
- `DeclMacro`. It was a less powerful duplicate of `NormalTT` (aka `LegacyBang` after renaming) and can be replaced by it.

Stuff like this slows down any attempts to refactor the expansion infra, so it's desirable to retire it already.
I'm not sure whether a lang team decision is necessary, but would be nice to land this sooner because I have some further work in this area scheduled.

The documentation commit (a9397fd0d5) describes how the remaining variants are different from each other and shows that there's actually some system behind them.

The last commit renames variants of `SyntaxExtension` in more systematic way.
- `ProcMacro` -> `Bang`
- `NormalTT` -> `LegacyBang`
- `AttrProcMacro` -> `Attr`
- `MultiModifier` -> `LegacyAttr`
- `ProcMacroDerive` -> `Derive`
- `BuiltinDerive` -> `LegacyDerive`

All the `Legacy*` variants are AST-based, as opposed to "modern" token-based variants.

src/test/ui/macros/nonterminal-matching.stderr

@@ -1,6 +1,9 @@
 error: no rules expected the token `enum E { }`
   --> $DIR/nonterminal-matching.rs:19:10
    |
+LL |     macro n(a $nt_item b) {
+   |     --------------------- when calling this macro
+...
 LL |     n!(a $nt_item b);
    |          ^^^^^^^^ no rules expected this token in macro call
 ...