Auto merge of #51474 - llogiq:from-docs, r=TimNN
Auto merge of #51474 - llogiq:from-docs, r=TimNN

add some docs to `From` conversions

This adds a helpful document to the bool → int* conversions as well as to the lossless integer conversions.

One of #51430 down, some more to go.

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