Add mention of --check-untyped-defs option to docs

[tonyelewis]

On my first try of mypy, I was disappointed that I couldn't find a way to get mypy to complain about this source:

def fn_a(x: int) -> None:
	pass

def fn_b():
	fn_a('silly argument')

I would have found it helpful if the No errors reported for obviously wrong code section had mentioned the --check-untyped-defs option. I suspect others might also benefit from adding this brief mention.

Thanks very much.

[tonyelewis]

Thanks very much.