Clearly define each category (e.g. host fields, event fields, network fields)

**Please describe what you are requesting**

While the schema documentation via https://schema.graylog.org/en/stable/schema/entities.html is very helpful, there isn't any explanation about the different field types, why they exist, how they are defined, what they are meant to be used for, what criteria goes into defining them (random example, what is a host field and what dictates what data populates host fields?)

I'm just noticing that some pages do have a note at the top, but not all.

Start by reviewing the schema documentation at schema.graylog.org/en/stable/schema/entities.html, including the existing category notes. Document what each field category means, why it exists, how it is defined, and what data belongs in it; done means all categories have consistent explanatory guidance.

Written by the indexing model from the issue text.