Sadly the best way to see what functions are available are to 1) read other lua code and 2) read the l_*.cc source files that define the clua libraries.
For 1), good repositories are
HilariousDeathArtists's lua and
my lua, among others you can find on the internet.
For the l_*.cc files in the source dir of the repo, look for a file named after a thing you're interesting it, like "l_you.cc" to see what's defined in the "you" library you mentioned for "you.mutation()". At the bottom of those files the functions are wrapped up into a structure that defines the library. You have to make sure that you're looking at the so-called "clua" (client lua) library definitions and not the so-called "dlua" (dungeon lua) libraries, since the former is what you can use from your RC and the latter can only be used by crawl's level generation system.