Database load balancing: Document how the additional section processing for SRV records work

The following discussion from !15691 (merged) should be addressed:

• @tkuah started a discussion:

  We should probably document how the additional processing works, and most importantly hostnames that don't resolve gets dropped.

  Also, I realise that this might drop SRV records that return IP adddresses in the answer section but it's not a valid use-case right now.