`FunctionalProperty` has no description in OpenAPI/TD

[rwb27]

Describe the bug
The docstring for functional properties is not being correctly displayed as the description in OpenAPI or Thing Description

To Reproduce
Look at OpenAPI or Thing Description docs for the latest OFM release (alpha5). Data properties are documented, but functional properties have no description or title.

Expected behaviour
The docstring of the getter should be used in the documentation.

We should fix this and add or fix the corresponding test code.