typing docs: Move some classes out of the "Generics" section (#104707)

- AnyStr can be used in type annotations, contrary to the section header
- Unpack can also be used in annotations, and its use is not restricted
  to generics. It makes more sense with other building blocks like Required.
- Protocol is not necessarily generic.

Also fix the indentation for two notes associated with Concatenate.

Split off from #104642, but I think this change is independently an
improvement.