Complement cheatsheet for Optionals (#6074)

Fixes #5037

Add a paragrah to explain there are 2 ways to make the typechecker
understand that an optional is not `None`: use a if condition or an
`assert`.

Author: vincent-prz

docs/source/cheat_sheet.rst

@@ -46,9 +46,12 @@ Built-in types
 
    # Use Optional[] for values that could be None
    x = some_function()  # type: Optional[str]
+   # Mypy understands a value can't be None in an if-statement
    if x is not None:
-      print x
-
+       print x.upper()
+   # If a value can never be None due to some invariants, use an assert
+   assert x is not None
+   print x.upper()
 
 Functions
 *********

docs/source/cheat_sheet_py3.rst

@@ -72,9 +72,12 @@ Built-in types
 
    # Use Optional[] for values that could be None
    x: Optional[str] = some_function()
+   # Mypy understands a value can't be None in an if-statement
    if x is not None:
-       print(x)
-
+       print(x.upper())
+   # If a value can never be None due to some invariants, use an assert
+   assert x is not None
+   print(x.upper())
 
 Functions
 *********