Rollup merge of #71692 - dfreese:cfgdocs, r=kennytm
Rollup merge of #71692 - dfreese:cfgdocs, r=kennytm

Add clarification on std::cfg macro docs v. #[cfg] attribute

The wording was discussed, to a limited degree in #71679.  This tries to
address some confusion I as well as someone else had independently when
looking at this macro.

Fixes #71679

aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust