David Turner 1be5bb5bfd Recognise direct buffers in heap size docs (#42070)
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
2019-05-10 13:56:47 +01:00
..