Guide: articulate the advantages of ownership over garbage collection.

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 
I don't think this is correct (cc @nikomatsakis).
"Lifetime errors" come from regionck, which infers appropriate lifetimes and enforces their coherence.
The borrowck produces, well, borrow errors.