Connect descriptions of writable nested serializers in the docs

[chreke]

There seems to be a bit of overlap in describing how to support writable nested serializers in the docs; there's a brief mention of how to do it in the Relations section and a more in-depth explanation in the Serializers section.

Links to the relevant sections:

• Relations: https://github.com/encode/django-rest-framework/blob/master/docs/api-guide/relations.md#writable-nested-serializers
• Serializers: https://github.com/encode/django-rest-framework/blob/master/docs/api-guide/serializers.md#dealing-with-nested-objects

For someone trying to implement nested model serializers it might be helpful to link from the Relations section to the relevant part of the Serializers section. (Another option might be to try to consolidate the two into a single subsection on writable nested serializers in the Serializers doc, and link to that from Relations.)

What do you guys think? I'd be happy to write a PR if you think this is a good idea