Improvement - Board API restrictions (time control) info at each relevant endpoint

Logging this issue based on discussion - https://discord.com/channels/280713822073913354/693123750845218836/1123948648896667728

It is easy to not read the _entire_ documentation... :sweat_smile:
Sometimes one skims the documentation and finds the endpoint which seems relevant, and only reads about that endpoint.

Repeating the Board API section notice about restrictions regarding Time Control,
on each endpoint which is affected by the Time Control restriction could help people from pulling their hair... :sweat_smile:

These are the endpoints I suspect involve Time Control when using the Board API / a token with "board:play" scope:

https://lichess.org/api#tag/Board/operation/apiBoardSeek
https://lichess.org/api#tag/Challenges/operation/challengeCreate
https://lichess.org/api#tag/Challenges/operation/challengeAi
https://lichess.org/api#tag/Challenges/operation/challengeAccept

Start with the Board API restriction notice and the four endpoint entries linked in the issue: apiBoardSeek, challengeCreate, challengeAi, and challengeAccept. Repeat the relevant Time Control restriction information at each affected endpoint, then verify that every listed endpoint documents the restriction clearly.

Written by the indexing model from the issue text.