[docs] __builtins__ propagation is misleading described in exec and eval documentation (issue 26363)

[vadmium+py at gmail.com]

https://bugs.python.org/review/26363/diff/19291/Doc/library/functions.rst
File Doc/library/functions.rst (right):

https://bugs.python.org/review/26363/diff/19291/Doc/library/functions.rst#newcode399
Doc/library/functions.rst:399: present and  does not contain a value for
the key '__builtins__',
What was wrong with the original? Your new wording raising questions
about whether you can have a key without a value, is None considered a
value, etc.

https://bugs.python.org/review/26363/