Document package type to use for modules primarily providing field types

Problem/Motivation

This is a follow up to #1255696: Needs docs update: Move field type modules into separate "Field type" package which was committed and re-opened for a documentation update. This issue is for completing that documentation work.

Update documentation to include information about the package to use in .info.yml for modules where the primary purpose is to provide a field type. The change record for original issue will be useful.

Steps to reproduce

Proposed resolution

Update documentation to include information about the package to use in .info.yml files for modules where the primary purpose is to provide a field type.

Remaining tasks

Determine where to add the documentation

User interface changes

API changes

Data model changes

Release notes snippet

Release notes snippet