Adds section to describe intended usage of `pipeline.buffer.type`

[andsel]

Release notes

[rn:skip]

PREVIEW: https://logstash_bk_16083.docs-preview.app.elstc.co/guide/en/logstash/master/jvm-settings.html

What does this PR do?

Adds section to describe the intended usage of and impact on memory sizing.

Why is it important/What is the impact to the user?

Give better insights when to use the pipeline.buffer.type and which is the expected impact on performance and memory consumption.

Checklist

• [x] My code follows the style guidelines of this project
• ~~[ ] I have commented my code, particularly in hard-to-understand areas~~
• [x] I have made corresponding changes to the documentation
• ~~[ ] I have made corresponding change to the default configuration files (and/or docker env variables)~~
• ~~[ ] I have added tests that prove my fix is effective or that my feature works~~

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Relates #16054

[karenzone]

@andsel, I left two suggestions inline. If you apply them both, that should clear the docs-ci issues. :-)