doc(library/init/data/nat/lemmas.lean): add docstring for nat.find (#566)
doc(library/init/data/nat/lemmas.lean): add docstring for nat.find (#566)

Users do tend to need this core Lean command occasionally, so I thought it was worth adding a docstring so once they've found it they can see how it works.



digama0/lean,digama0/lean,leanprover-community/lean,digama0/lean,leanprover-community/lean,digama0/lean,leanprover-community/lean,digama0/lean,leanprover-community/lean,leanprover-community/lean,digama0/lean,leanprover-community/lean