M-x customize-apropos some-string

This is how I've discovered most things outside of just reading the documentation (and usually I go to the documentation once I find it). It's also helpful that the info pages in emacs are also searchable with:

  M-x info-apropos some-string

Both are kind of firehoses, but C-s inside the resulting buffer can usually narrow it down pretty quickly.

I'd like to know what other editors have a discovery system like this built-in.