Add "msg" argument to new allowances.

[shawnbrown]

Add msg=None argument to new allowances.

In cases where msg is None, but a filter function has a __doc__, use the docstring as the message.

[shawnbrown]

This is done:

• Initial implementation: 1854a6cbb870d5c9457004a2498fa84b4abc45d3
• Cleaned-up: 36f596aac47178cfe991b57953a7a4b1b6ea6ea8