Cross-reference doc chapter from testing chapter
Cross-reference doc chapter from testing chapter

We don't completely cover documentation tests in the testing chapter,
since we cover them in the documentation chapter. So make sure people
know that.

Fixes #28082

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