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

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#ne... 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/