add docs for configuring webhook URL from env var

docs/graphql/manual/event-triggers/create-trigger.rst

@@ -11,7 +11,8 @@ Event triggers can be created using the Hasura console.
 Open the Hasura console, head to the ``Events`` tab and click on the ``Create trigger`` button to open up the
 interface below to create an event trigger:
 
-.. image:: ../../../img/graphql/manual/event-triggers/create-event-trigger-annotations.png
+.. image:: ../../../img/graphql/manual/event-triggers/create-event-trigger.png
+   :scale: 75 %
 
 Parameters
 ----------
@@ -27,19 +28,26 @@ The postgres schema and table name on which event trigger needs to be created.
 
 **Operations**
 
-The operation on which event trigger needs to be invoked. Can be INSERT, UPDATE and/or DELETE.
+The operation on which event trigger needs to be invoked. Can be ``INSERT``, ``UPDATE`` and/or ``DELETE``.
 
 **Webhook URL**
 
-The HTTP(s) URL which will be called with event payload on configured operation. Must be a POST handler.
+The HTTP(s) URL which will be called with event payload on configured operation. Must be a ``POST`` handler. This URL
+can be entered manually or can be picked up from an environment variable (*the environment variable needs to be set
+before using it for this configuration*).
 
 Advanced Settings
 -----------------
 
+.. image:: ../../../img/graphql/manual/event-triggers/create-event-trigger-advanced-settings.png
+   :scale: 75 %
+
+
 Listen columns for update
 ^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Update operations are special because you may want to trigger webhook only if specific columns have changed in a row. Choose the columns here which you want the update operation to listen on.
+Update operations are special because you may want to trigger webhook only if specific columns have changed in a row.
+Choose the columns here which you want the update operation to listen on.
 
 If a column is not selected here, then an update to that column will not trigger webhook.
 
@@ -61,5 +69,7 @@ Custom headers can be added to an event trigger. Each webhook request will have
 Each header has 3 parameters:
 
 1. ``Key``: Name of the header e.g. Authorization or X-My-Header.
-2. ``Type``: One of ``static`` or ``from env variable``. ``static`` means the value provided in the ``Value`` field is the raw value of the header. ``from env variable`` means the value provided in the ``Value`` field is the name of the environment variable in the GraphQL Engine which will be resolved before sending the header.
+2. ``Type``: One of ``static`` or ``from env variable``. ``static`` means the value provided in the ``Value`` field is
+   the raw value of the header. ``from env variable`` means the value provided in the ``Value`` field is the name of
+   the environment variable in the GraphQL Engine which will be resolved before sending the header.
 3. ``Value``: The value of the header. Either static value or name of an environment variable.