docs: modify update-message-flag to contain information about flags.

[Hypro999]

#12092

[Hypro999]

@timabbott, all that's left is just filling in the blanks 🙂

[timabbott]

Nice, merged as c6b0343, after adding fa77467 and clarifying what belongs here.

Thinking about how this could be better, I think we should probably add a link to that section from other articles that interact with message flags (primarily the GET /messages endpoint, which returns that day).

@Hypro999 can you take care of that in a follow-up commit?

[Hypro999]

@timabbott: Done.