[Doc]Refine recommendations for heap settings

[karenzone]

Revisit and refine this statement:

"The recommended heap size for typical ingestion scenarios should be no less than 4GB and no more than 8GB."

From review comments in #11867: "I am not sure about these 4/8 numbers. For me 'typical ingestion scenarios' seems too vague and 4/8 seems somewhat arbitrary..."

Topic: https://www.elastic.co/guide/en/logstash/7.8/jvm-settings.html#heap-size Source: /docs/static/config-details.asciidoc

[toby-sutor]

Apart from that, I think that the problem of compressed pointers being disabled at 32+ GB is the same with Logstash as it is with Elasticsearch (ref. https://www.elastic.co/guide/en/elasticsearch/reference/current/heap-size.html). Thus, I think we should mention in the documentation to not go above 32 GB of JVM heap.

[colinsurprenant]

Good point @toby-sutor .

As for the 4/8 suggestion, I still believe these are arbitrary, could we instead spin it with something like «for typical ingestion scenarios a heap size of 4 to 8GB has proven to be sufficient» and then continue with the other point we could merge here «You can make more accurate measurements of the JVM heap by using either the jmap command line utility distributed with Java or by using VisualVM. For more info, see Profiling the Heap.»

Some in essence we say "4 to 8 seems typically right but we encourage you to measure and adjust accordingly".

WDYT? /cc @jsvd

[milanage]

We've recently received some user feedbacks regarding this statement - are we still going to improve it?