Document the two main experimental performance flags
Document the two main experimental performance flags

Add documentation for the -sassertNoSlicing=true and -snoRefCount=true
flags, along with explanations for why they're available, what
limitations they come with, and work that's currently underway to
render them unnecessary.

hildeth/chapel,chizarlicious/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,hildeth/chapel,CoryMcCartan/chapel,chizarlicious/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,CoryMcCartan/chapel,chizarlicious/chapel,chizarlicious/chapel,CoryMcCartan/chapel,chizarlicious/chapel,chizarlicious/chapel,hildeth/chapel,hildeth/chapel,hildeth/chapel