Auto merge of #29498 - wthrowe:replace-pattern, r=alexcrichton

It appears this was left out of RFC rust-lang/rfcs#528 because it might be useful to
also generalize the second argument in some way.  That doesn't seem to
prevent generalizing the first argument now, however.

This is a [breaking-change] because it could cause type-inference to
fail where it previously succeeded.

Also update docs for a few other methods that still referred to `&str` instead of patterns.

diff --cc src/libcollections/str.rs
Simple merge