Clarify the absence of fields with nested messages

There are plenty of fields of message type. I propose to explicitly write in docs whether the field MUST be set.

Where it is not written otherwise, I suggest to require field presence. Later we can relax the requirement, which would be a backwards compatible change.

These requirements will reduce the number of uncertainties when writing software.

Note: this only applies to fields in requests/responses.

Start by locating the documentation for request and response fields whose types are nested messages. Review how field presence is currently described, then document whether each such field must be set unless stated otherwise; done means the presence rule is explicit throughout the relevant documentation.