Parallel execution changed the way bloom queries are executed, so update
the EXPLAIN output, and restructure the docs to be clearer and more
accurate.

Reported-by: Daniel Westermann

Discussion: https://postgr.es/m/ZR0P278MB0122119FAE78721A694C30C8D2340@ZR0P278MB0122.CHEP278.PROD.OUTLOOK.COM

Author: Daniel Westermann and me

Backpatch-through: 9.6