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().)

Doc/library/functions.rst

@@ -435,8 +435,10 @@ are always available.  They are listed here in alphabetical order.
    The *expression* argument is parsed and evaluated as a Python expression
    (technically speaking, a condition list) using the *globals* and *locals*
    dictionaries as global and local namespace.  If the *globals* dictionary is
-   present and lacks '__builtins__', the current globals are copied into *globals*
-   before *expression* is parsed.  This means that *expression* normally has full
+   present and does not contain a value for the key ``__builtins__``, a
+   reference to the dictionary of the built-in module :mod:`builtins` is
+   inserted under that key before *expression* is parsed.
+   This means that *expression* normally has full
    access to the standard :mod:`builtins` module and restricted environments are
    propagated.  If the *locals* dictionary is omitted it defaults to the *globals*
    dictionary.  If both dictionaries are omitted, the expression is executed in the