Document pool status on websocket subscriptions

[stefangutica]

Description of the pull request (what is new / what has changed)

Documents the pool status on websocket subscriptions, in the Pool (Pulse) section of ws-subscriptions.md:

• poolUpdate now carries a status: success, tooLarge or internalServerError. Added to the existing update example, which also gets its poolCount indentation fixed.
• New table with the poolUpdate fields:
  • status;
  • pool, null unless the status is success;
  • poolCount, the total count of the pool, regardless of type, when the status is not success, or null when it could not be counted either.
• New "Pool too large" and "Pool unavailable" examples: in both cases the update carries no transactions, only the total count, and clients should show a message instead of the list.

This documents the behaviour added in mx-api-service by the pool-count-from-gateway PR, so it should be merged together with or after that PR.

Did you test the changes locally ?

• yes
• no

Did you regenerate static/llms.txt and static/llms-full.txt ? (happens at build time)

• yes
• no

Which category (categories) does this pull request belong to?

• document new feature
• update documentation that is not relevant anymore
• add examples or more information about a component
• fix grammar issues
• other