[alerting] adds doc on JSON-expanded action variables and task manage…

…r max_workers (#92720)

resolves #90006

For task manager, adds a note about the fact that the max_workers will be
limited to 100 starting in 8.0.  Currently we allow any value (because we
always have), but do print a "deprecation" warning that the limit cannot
be exceeded starting in 8.0

For alerting, adds note about the JSON expansion of action variables which are objects.

docs/settings/task-manager-settings.asciidoc

@@ -27,6 +27,6 @@ Task Manager runs background tasks by polling for work on an interval.  You can
 
   | `xpack.task_manager.max_workers`
   | The maximum number of tasks that this Kibana instance will run simultaneously.  Defaults to 10.
-
+    Starting in 8.0, it will not be possible to set the value greater than 100.
 
 |===

docs/user/alerting/defining-alerts.asciidoc

@@ -95,6 +95,10 @@ Some cases exist where the variable values will be "escaped", when used in a con
 
 Mustache also supports "triple braces" of the form `{{{variable name}}}`, which indicates no escaping should be done at all.  Care should be used when using this form, as it could end up rendering the variable content in such a way as to make the resulting parameter invalid or formatted incorrectly.
 
+Each alert type defines additional variables as properties of the variable `context`.  For example, if an alert type defines a variable `value`, it can be used in an action parameter as `{{context.value}}`.  
+
+For diagnostic or exploratory purposes, action variables whose values are objects, such as `context`, can be referenced directly as variables.  The resulting value will be a JSON representation of the object.  For example, if an action parameter includes `{{context}}`, it will expand to the JSON representation of all the variables and values provided by the alert type.
+
 You can attach more than one action. Clicking the "Add action" button will prompt you to select another alert type and repeat the above steps again.
 
 [role="screenshot"]