memquota: be more firm about avoiding panics, and tidy up docs

1. Move the handful of things from memory-limit.md that aren't covered into the proper docs, into said proper docs.
2. Use clippy to ensure there aren't any arithmetic overflow panics
3. Delete obsolete text in memory-limit.md
4. Copy plans for how to apply this all to concrete queues, from an MR discussion, to in-tree docs