Docs about default value for dotted source, additional tests

[rpkilby]

WIP changes to docs, and additional tests for behavior of source w/ dotted notation.

Closes #5488.

Waiting for a nested ModelSerializer example from @xordoquy.

Also, is the current exception sufficient, or do we need to mention the necessity of a default value?
eg, this serializer:

class ColleagueUserSerializer(BaseModelSerializer):
    city = serializers.IntegerField(source='profile.contact.address.city')

generates this exception:

AttributeError: Got AttributeError when attempting to get a value for field `city` on serializer `ColleagueUserSerializer`.
The serializer field might be named incorrectly and not match any attribute or key on the `User` instance.
Original exception text was: 'NoneType' object has no attribute 'address'.

[carltongibson]

@rpkilby What's the status here? (Ta!)

@xordoquy Can you review and advise? Are you happy that #5375 was correct? Do we need more here?

I'm going to look at getting the Django 2.0 compat (actually) in place and then roll a 3.7.1. That'll be next week so there's plenty of time here.

[xordoquy]

Well, I think it's correct.
I'm just not so happy that a missing default values leads to an attribute error with ModelSerializers with DRF 3.7 but I couldn't find the time to write a test case out of it.

[rpkilby]

@xordoquy, if you have a simplified example of the nested ModelSerializer setup, I could work on turning that into a test case.

[carltongibson]

I'm going to go with this as is. (We can always come back again.)