New compiler flag :elide-asserts documentation

See:

- https://groups.google.com/forum/#!topic/clojure/bCNXDBrDVEs
- https://github.com/clojure/clojurescript/blob/905c64445faa1c60e21b66fb226982759b0d4001/src/clj/cljs/closure.clj#L1134