civimail.md - Collapse "HTML vs Text" section

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.

docs/framework/civimail.md

@@ -23,6 +23,7 @@ CiviCRM's [token functionality](https://docs.civicrm.org/user/en/latest/common-w
 - **Performance**: Divide mail-composition into batches and split the batches among parallel workers. Moreover, when processing each batch, improve efficiency by minimizing the #SQL queries - i.e. fetch all records in a batch at once, and only fetch columns which are actually used.
 - **Security**: Do not trust email authors with a fully programmable language.
 - **Contact Records**: The main data for mail-merge came from contact records. Other data (contribution, event, participant, membership, etc) were not applicable.
+- **Adaptive Text/HTML**: Email messages often have two renditions, `text/plain` and `text/html`. Some tokens, such as `{domain.address}`, may present different formatting in each medium. Other tokens, such as `{action.unsubscribe}`, can even present a different user-experience.
 
 Over time, the token functionality evolved:
 
@@ -166,20 +167,6 @@ Some example tokens and their meaning
 
 For more examples of tokens and token replacement see the [Token documentation](https://wiki.civicrm.org/confluence/display/CRMDOC/Tokens)
 
-### HTML vs Text
-
-Since we support both HTML and Text formatting of outgoing mail, we will need rules for how the tokens are used in both cases. This is mainly an issue for action tokens, but certain other tokens (such as **domain.address**) may be formatted differently in HTML mode.
-
-In HTML content, action tokens should always be used as if they were URLs. If the action is an email address, a mailto: prefix will be added automatically
-
-```
-Example HTML
-To unsubscribe from this mailing list, click <a href="{action.unsubscribe}">here</a>.
-
-Example text
-Send email to {action.unsubscribe} to unsubscribe from this mailing list.
-```
-
 ### NULL values and Defaults
 
 Many non-required contact fields are exposed to the token processor, and not every contact will have values for every token. By default, if there is no value for a token field, it is replaced with a blank string. To override this behavior, use either Smarty or `hook_civicrm_tokens()` and `hook_civicrm_tokenValues()`. The following is an example of using Smarty to override a token (note that since Smarty sees tokens as constants, they will never register as empty. The `{capture assign=variable}{token}{/capture}` is a workaround for this):