Define what the key is (#1825)
Define what the key is (#1825)

* Define what the key is

The article said "Mnesia support indices on non-key columns", but hadn't mentioned key columns yet.

* Address PR feedback

doomspork/elixir-school,doomspork/elixir_school,doomspork/elixir-school,nscyclone/elixir-school,nscyclone/elixir-school,nscyclone/elixir-school,doomspork/elixir_school,doomspork/elixir-school,doomspork/elixir_school,nscyclone/elixir-school