Rollup merge of #31329 - quodlibetor:no-const-doc-in-stable, r=alexcrichton
Rollup merge of #31329 - quodlibetor:no-const-doc-in-stable, r=alexcrichton

Fixes #31098

AFAICT this is the only place where rustdoc explicitly checks if we are on stable before emitting content, so I can't tell if this is the sane way to handle this, or if anything else should be done to make sure that nobody forgets to remove this check when `const` is stabilized.

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