Rollup merge of #45099 - mikeyhew:fix-astconv-self-type-comments, r=nikomatsakis
Rollup merge of #45099 - mikeyhew:fix-astconv-self-type-comments, r=nikomatsakis

Update comments referring to old check_method_self_type

I was browsing the code base, trying to figure out how #44874 could be implemented, and noticed some comments that were out of date and a bit misleading (`check_method_self_type` has since been renamed to `check_method_receiver`). Thought it would be an easy first contribution to Rust!

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