Rollup merge of #55874 - denisvasilik:docs, r=alexcrichton
Rollup merge of #55874 - denisvasilik:docs, r=alexcrichton

string: Add documentation for `From` impls

Hi this is part of #51430. I'm a first time contributor, so I started with a small task adding a bit of documentation for From impls.

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