Cover `omit_defaults` handling in `asdict` and `to_builtins` documentation

[ncoghlan]

Description

(I'm not sure if this is a bug report or a feature request, so I opted for a feature request in case the current behaviour is intentional)

I am using msgspec to help communicate with a server where a boolean field can be set to None in the data model not because null is a valid value, but because it is permitted to omit the key entirely. The relevant struct has omit_defaults=True set accordingly.

However, msgspec isn't handling the full message serialisation (httpx is doing that), so I attempted to use asdict to generate the message spec portion of the message.

This failed, as asdict includes all fields, without paying attention to omit_defaults.