Rollup merge of #148447 - estebank:outer-param-2, r=jackh726

Tweak E0401

More accurate span pointing at use place of outer param (at ident instead of full path).

Add note explaining why outer item params can't be used in inner item.

Use structured suggestion for what `Self` should have been.

Follow up to rust-lang/rust#148370.

Fix rust-lang/rust#37892.