Pretty up Slack formatting. (#15898)

* Pretty up Slack formatting.

* Normalize spaces by turning every instance of two or more spaces into
  one space.
* Allow for custom URL links even when strip_tags() is in effect by
  transformaing "standard" markdown syntax into Slack link markdown
  after strip_tags() is run.

* Document the changes to the Slack transport.

While here, flesh out the Slack docs to match the configuration options
one currently sees in the transport.

* Use backticks to escape things from markdown rendering.

* We don't need this other escaping now.

* Fix custom link due to line break.

* Fix spelling typo.

---------

Co-authored-by: Joe Clarke <jclarke@cisco.com>

doc/Alerting/Transports.md

@@ -774,16 +774,36 @@ only required value is for url, without this  then no call to Slack will be made
 
 We currently support the following attachment options:
 
-`author_name`
+- `author_name`
+
+We currently support the following global message options:
+
+- `channel_name` : Slack channel name (without the leading '#') to which the alert will go
+- `icon_emoji` : Emoji name in colon format to use as the author icon
 
 [Slack docs](https://api.slack.com/docs/message-attachments)
 
+The alert template can make use of
+[Slack markdown](https://api.slack.com/reference/surfaces/formatting#basic-formatting).
+In the Slack markdown dialect, custom links are denoted with HTML angled
+brackets, but LibreNMS strips these out. To support embedding custom links in alerts,
+use the bracket/parentheses markdown syntax for links.  For example if you would
+typically use this for a Slack link:
+
+`<https://www.example.com|My Link>`
+
+Use this in your alert template:
+
+`[My Link](https://www.example.com)`
+
 **Example:**
 
 | Config | Example |
 | ------ | ------- |
 | Webhook URL | <https://slack.com/url/somehook> |
-| Slack Options | author_name=Me |
+| Channel | network-alerts |
+| Author Name | LibreNMS Bot |
+| Icon | `:scream:` |
 
 ## SMSEagle