refactor: rename pre-restore trigger to scheduler-pre-restore

This better reflects what the trigger does, and allows us to use the pre-restore trigger at the beginning of the ps:restore command.

docs/development/plugin-triggers.md

@@ -1753,7 +1753,7 @@ APP="$1"; IMAGE_TAG="$2"; IMAGE=$(get_app_image_name $APP $IMAGE_TAG)
 
 - Description: Allows you to run commands before all containers are restored
 - Invoked by: `dokku ps:restore`
-- Arguments: `$DOKKU_SCHEDULER $APP`
+- Arguments:
 - Example:
 
 ```shell
@@ -2187,6 +2187,25 @@ DOKKU_SCHEDULER="$1"; APP="$2";
 # TODO
 ```
 
+### `scheduler-pre-restore`
+
+> Warning: The scheduler plugin trigger apis are under development and may change
+> between minor releases until the 1.0 release.
+
+- Description: Allows you to run commands before an app is restored
+- Invoked by: `dokku ps:restore`
+- Arguments: `$DOKU_SCHEDULER $APP`
+- Example:
+
+```shell
+#!/usr/bin/env bash
+
+set -eo pipefail; [[ $DOKKU_TRACE ]] && set -x
+DOKKU_SCHEDULER="$1"; APP="$2";
+
+# TODO
+```
+
 ### `scheduler-post-delete`
 
 > Warning: The scheduler plugin trigger apis are under development and may change