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 it is particularly important to be too pedantic about the internal compiler passes, but maybe this sentence could be rephrased to just "it will report an error and your program will refuse to compile".