Rollup merge of #47145 - frewsxcv:frewsxcv-linewriter-error, r=QuietMisdreavus
Rollup merge of #47145 - frewsxcv:frewsxcv-linewriter-error, r=QuietMisdreavus

Document when LineWriter flushes; document errors for into_inner.

Fixes https://github.com/rust-lang/rust/issues/42468.

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