This commit slightly reworks the recommendations in the docs about setting the
heap size:
* the "rules of thumb" are actually instructions that should be followed
* the reason for setting `Xmx` to 50% of the heap size is more subtle than just
leaving space for the filesystem cache
* it is normal to see Elasticsearch using more memory than `Xmx`
* replace `cutoff` and `limit` with `threshold` since all three terms are used
interchangeably
* since we recommend setting `Xmx` equal to `Xms`, avoid talking about setting
`Xmx` in isolation
Relates #41954
1be5bb5bfd