Standardize some usages of "which" in docstrings

In US english, "that" is used in restrictive clauses in place of
"which", and often affects the meaning of sentences.

In UK english and many dialects, no distinction is
made.

While Rust devs want to avoid unproductive pedanticism, it is worth at
least being uniform in documentation such as:

http://doc.rust-lang.org/std/iter/index.html

and also in cases where correct usage of US english clarifies the
sentence.