Document GUILD_CREATE with an unavailable guild scenario

[Mehgugs]

The recent outage highlighted to a couple of users that it is possible to receive a GUILD_CREATE with an unavailable guild object as its payload. It is unclear to me what scenario this would indicate but from a brief conversation in ddevs it seems that it can occur if you were added to a guild during / shortly before an outage, and similar cases where the guild was not in the ready payload initially.

This section of the docs should probably note that this is possible.