Document performance gains from property tokens

Problem/Motivation

Per #3039650-32: Metatag discards cacheability metadata, results in a performance hit:

Are you using any special tokens or so with special cacheability? Are you using property tokens ([node:field:value] instead of [node:field])? That can massively speed up token generation because the non-property tokens actually render the field into HTML through a formatter and then remove that again.

Steps to reproduce

Proposed resolution

Locate proper place to document this performance hit and fix (within module help text? within documentation? within README?).
Write and place documentation.

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet