Document macro_use shadow restriction

There seems to be a restriction that macro_use cannot shadow a macro_use macro when expanded from within a macro_rules macro.

#[macro_use(foo)] extern crate m;

macro_rules! m1 { () => {
    #[macro_use(foo)] //~ ERROR `foo` is already in scope
    extern crate m as _;
}}
m1!();

emits:

error: `foo` is already in scope
  ╭▸ src/lib.rs:4:17
  │
4 │     #[macro_use(foo)] //~ ERROR `foo` is already in scope
  │                 ━━━
  ‡
7 │ m1!();
  │ ───── in this macro invocation
  │
  ├ note: macro-expanded `#[macro_use]`s may not shadow existing macros (see RFC 1560)
  ╰ note: this error originates in the macro `m1` (in Nightly builds, run with -Z macro-backtrace for more info)

I couldn't find a rule (or combination of rules) that would cover this situation.

Start with the provided macro_rules!/macro_use example and the diagnostic's reference to RFC 1560. Locate the Rust Reference section covering macro_use and document the applicable shadowing restriction, including whether macro-expanded attributes are covered; done when the example's behavior and rule are clearly explained.