docs: Make embedded fields nullable by default

docs: Make embedded fields nullable by default

As discussed at [1], "subtypes can add restrictions, but they cannot
relax restrictions that are already in place." These fields need to be
marked nullable and then "subclassed" to set non-nullability if
required.

[1] https://github.com/OAI/OpenAPI-Specification/issues/1368

Signed-off-by: Stephen Finucane <stephen@that.guru>