api: expose rate control params via x265_param_parse() and CLI

Adds range checks for qCompress, which has documented limits. The others have
very minimal explanations; so I'm not adding them to the CLI help. Users should
not touch them unless they know what they are doing.

Note this commit doesn't bump X265_BUILD since no new params were added.
cli: cleanup CLI help, add 'verbose' tier

Remove a lot of uncommon features from the initial help output, require
--log-level debug --help to see it all
api: cleanup comments
docs: document RC params, at least minimally