Fix poll_message_set API documentation to consistent with Python style
Reviewed By: Blendify
Differential Revision: https://developer.blender.org/D12150
Python scripts can now define the reason it's poll function fails using:
`Operator.poll_message_set(message, ...)`
This supports both regular text as well as delaying message creation
using a callback which should be used in situations where constructing
detailed messages is too much overhead for a poll function.
Ref D11001
