Rollup merge of #40997 - donniebishop:from_utf8_linking, r=steveklabnik

Added links to types in from_utf8 description

References #29375. Link to types mentioned in the documentation for `from_utf8` (`str`, `&[u8`], etc). Paragraphs were reformatted to keep any one line from being excessively long, but are otherwise unchanged.

diff --cc src/libcore/str/mod.rs
Simple merge