MySQL integration: add ECS fields to docs

[mtojek]

Reported in #313

One thing missing here is the updating the fields.yml files. The problem becomes obvious with the nginx access logs. Historically there was just one massive fields.yml for Filebeat where all the fields were shipped together and we had a tree of definitions: Global, ECS, Module, Fileset. Now that we have a template per Dataset, all the fields used must be defined on the Dataset level. This has the advantage that the template becomes much more compact but brings the challenge, there is no easy way to tell which fields from Global, ECS etc. are used in the dataset. We can't just take all of ECS as this is too many fields and not all are used.
Having it documented separately also solves an other issue: It allows us to document how an ECS fields is exactly used in a dataset. What does source.ip exactly mean in the context of nginx.access logs. So far, we only had a generic doc but now we can fill in the details.

[mtojek]

Resolving. Merged.