bpo-22057: Clarify eval() documentation (GH-8812)

If a globals dictionary without a '__builtins__' key is passed to
eval(), a '__builtins__' key will be inserted to the dictionary:

    >>> eval("print('__builtins__' in globals())", {})
    True

(As a result of this behavior, we can use the builtins
print() and globals() even if we passed a dictionary without a
'__builtins__' key to eval().)