These documents are misleading -- they talk about CiviMail because, *historically*,
much of this grew out of CiviMail. However, *today*, tokens are used more generally.
Also, in `civicrm-core`, `TokenProcessor` is *not* used by CiviMail. (You have to
install FlexMailer to use it.)

The old section signal a few things:

1. "CiviMail design needs to handle HTML/Text" - but this can be more concisely stated as part of he requirements summary. Added a blurb there.
2. "The `{action.unsubscribe}` is used differently in each context" - but this is better explained as part of the *User Guide* for CiviMail.

Arguably, in the context of CiviMail developer docs, we should also signal "How to manage token content with text-vs-html", but this section
doesn't actually do that, and that's already signalled in the examples of extending the token system.

Discussed on `dev` with Seamus and Mathieu.  None of us could make any sense
of it.  It feels like an old spec pre-dating the actual writing of CiviMail.

related:
https://github.com/civicrm/civicrm-user-guide/pull/232

Resolve #316

Fixed typo: transacftion, now: transaction

(They don't show up as sub-lists unless you use 4)