Clarify docs and slightly optimize String.contains?/2
Clarify docs and slightly optimize String.contains?/2

  1. Change the docs for `contains?` to explicitly mention
     the verb "Searches"

  2. Add a note about using `Enum.member?/2` for membership tests

  3. Slightly optimize `String.contains?/2` when the string
     is too small

Closes #11611.

joshprice/elixir,elixir-lang/elixir,michalmuskala/elixir