Rollup merge of #48210 - jacob-hughes:clarify_sized_trait_in_api_docs, r=QuietMisdreavus
Rollup merge of #48210 - jacob-hughes:clarify_sized_trait_in_api_docs, r=QuietMisdreavus

Clarified why `Sized` bound not implicit on trait's implicit `Self` type.

This part of the documentation was a little confusing to me on first read. I've added a couple lines for further explanation. Hopefully this makes things a bit clearer for new readers.

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