[docs] Add note about expanding query planner heap size

[smyrick]

This is an experimental variable and will be removed in the future but it has been used a number of time now and is worth documenting

[router-perf[bot]]

CI performance tests

• [x] step - Basic stress test that steps up the number of users over time
• [ ] reload - Reload test over a long period of time at a constant rate of users
• [ ] step-with-prometheus - A copy of the step test with the Prometheus metrics exporter enabled
• [ ] events_without_dedup_callback - Stress test for events with a lot of users and deduplication DISABLED using callback mode
• [ ] events - Stress test for events with a lot of users and deduplication ENABLED
• [ ] events_without_dedup - Stress test for events with a lot of users and deduplication DISABLED
• [ ] events_big_cap_high_rate_callback - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity using callback mode
• [ ] xlarge-request - Stress test with 10 MB request payload
• [x] const - Basic stress test that runs with a constant number of users
• [ ] events_big_cap_high_rate - Stress test for events with a lot of users, deduplication enabled and high rate event with a big queue capacity
• [ ] step-jemalloc-tuning - Clone of the basic stress test for jemalloc tuning
• [ ] xxlarge-request - Stress test with 100 MB request payload
• [ ] demand-control-uninstrumented - A copy of the step test, but with demand control monitoring enabled
• [ ] no-graphos - Basic stress test, no GraphOS.
• [ ] events_callback - Stress test for events with a lot of users and deduplication ENABLED in callback mode
• [ ] large-request - Stress test with a 1 MB request payload
• [ ] demand-control-instrumented - A copy of the step test, but with demand control monitoring and metrics enabled

[abernix]

This was a PR that we should have probably landed a while ago. At this point, we'll opt to leave this out as we're looking at a deprecation route for it.