auto merge of #18176 : jkleint/rust/guide-borrow-wording, r=steveklabnik

Explain the primary disadvantage of garbage collection is runtime
overhead and unpredictable pauses.  Elucidate where the name "race
condition" comes from.  Emphasize that Rust can guarantee your code is
free of race conditions and other memory errors, with no runtime
overhead.

cc @steveklabnik

diff --cc src/doc/guide.md
Simple merge