Document the effect of `@[expose]`/`@[no_expose]` on each declaration

[YaelDillies]

What question should the reference manual answer?

There are two separate but related issues with https://lean-lang.org/doc/reference/latest/Source-Files-and-Modules/#module-scopes currently:

1. @[no_expose] isn't documented
2. @[expose] is only documented in relation to def, leaving implicit what @[expose]/@[no_expose] do to an abbrev. instance, structure, class, in particular what the default exposition is for each of these.

Additional context

Zulip