Document `unrestricted_attribute_tokens`

[ehuss]

• Version: 1.34
• Stabilization: Stabilize unrestricted_attribute_tokens rust#57367
• Tracking: Tracking issue for arbitrary token streams in non-macro attributes (unrestricted_attribute_tokens) rust#55208

[flying-sheep]

An aspect that hasn’t become clear to me: Are macros expanded or is macro syntax simply passed to on?

[petrochenkov]

@flying-sheep
Macro invocations are simply passed on, exactly like with proc macros.

Proc macros will need some mechanism like rust-lang/rfcs#2320 to interpret macro invocations in both their input and their helper attributes.