Rollup merge of #57452 - steveklabnik:improve-formatter-docs, r=frewsxcv
Rollup merge of #57452 - steveklabnik:improve-formatter-docs, r=frewsxcv

Improve docs for Formatter

Some improvements to `std::fmt::Formatter` to make it a bit more consistent with other documentation, as well as calling out that you don't ever instantiate one yourself.

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