Use docstring as schema description

[antonagestam]

See #202 (comment).

[antonagestam]

@apirogov Would you like to work on this feature?

[apirogov]

I digged a bit into the source and at looks to me that the __schema__ definitions everywhere just override the __schema__ method in SchemaField, and the only place where its connected to pydantic is __modify_schema__ in SchemaField, using that __schema__.

If this is the case, then would it not be enough to simply adapt __modify_schema__, so that it first getting the dict for the class returned by __schema__, and override / add the description from cls.__doc__, if it is defined?

If that's about it and I did not miss anything substantial, I surely can do it.

[antonagestam]

@apirogov I think that's it, should probably go next to the default for "title" here:

phantom-types/src/phantom/schema.py

Line 56 in fcb136d

return {"title": cls.__name__}

A few tests will probably also need to be updated.