Document `assert x.y is not None` in the cheat sheet

[JukkaL]

Asserting when the inferred type is Optional[X] but we expect just X is a common thing and may not be obvious to users.

[sp1rs]

This is already specified in the cheat sheet. This issue can be closed.
From cheat_sheet.rst

# Use Optional[] for values that could be None
x = some_function()  # type: Optional[str]
if x is not None:
   print x

[ilevkivskyi]

This is different. I think the idea of this issue is to document that there are two ways (which is important): assert x.y is not None and if x.y is not None.