Helper attributes in attribute macros

[extrawurst]

This little gotcha took me a while to figure out, maybe others will benefit from this addition swell

[tesuji]

Suggested change

	Since attribute macros can completely discard the input token stream filtering out custom `helper` attributes is entirely the job of the attribute macro.
	Since attribute macros can completely discard the input token stream filtering
	out custom `helper` attributes is entirely the job of the attribute macro.

[Centril]

I do not understand this addition; until we have something like rust-lang/rust#65823 there is no such thing in the language definition as "attribute macro helper attributes". (N.B. the reference is not a user guide -- it is an incomplete definition of the language).

[extrawurst]

Thanks for pointing me to that issue, its basically the same reason for my edit: new comers might be confused how to get the same behaviour like the helper attributes in a full fledged proc macro attribute, not realising that it is the macro's job to actually strip those out (or what ever else for that matter) - I get that it is weird to put something in the docs that looks like a user-guide thing but clearly I am not the only person stumbling over this...

[Centril]

@extrawurst I think adding this sort of documentation would be appropriate for e.g. a book on procedural macros or some other sort of tutorial (cc @dtolnay for possible locations). But people stumbling over things is not a good reason to add help notes to the reference, which is not teaching material.

[extrawurst]

fair enough