Rollup merge of #38497 - QuietMisdreavus:rustdoc-where-again, r=steveklabnik
Rollup merge of #38497 - QuietMisdreavus:rustdoc-where-again, r=steveklabnik

rustdoc: properly calculate line length for where clauses

Apparently, while I was cleaning up #37190, I regressed the formatting for long where clauses, where it wouldn't take the "prefix" length into account when deciding whether to break the line up. This patch fixes that.

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