Component documentation

Created the list of existing components in users_guide.rst. Please feel
free to downgrade from the “apparently working” list to “here be
dragons” any you think are appropriate. Or remove the link and just
list it under “Under development”, if you’d prefer no-one tries to use
it yet (or remove them entirely, if you’d rather keep your powder
completely dry).

Tweaked documentation in fastscape_stream_power.