options: Document parse_opt_val
options: Document parse_opt_val

Suggested-By: da39a3ee5e6b4b0d3255bfef95601890afd80709@msprotz

A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar,A-Manning/FStar,FStarLang/FStar