If you want to know what a command does the C-h k is the right tool to use. But what if you want to find out what a complex command can include as completion? As an example consider M-g g (goto-line): in this case the command is complex and the prefix is M-g. If you want to see what commands are tied to M-g something you can do:
M-g C-h
and this works for many of the complex commands.