add documentation for alterAdminPanel hook

docs/hooks/hook_civicrm_alterAdminPanel.md

+# hook_civicrm_alterAdminPanel
+
+## Summary
+
+This hook is invoked after all the panels and items on Administer CiviCRM screen have been
+generated and allows for direct manipulation of these items and panels.
+
+## Definition
+
+    hook_civicrm_alterAdminPanel(&adminPanel)
+
+## Parameters
+
+-   array adminPanel - array of panels on Adminster CiviCRM screen
+
+## Returns
+
+## Example
+
+    /**
+     * Alter panels on administer CiviCRM screen
+     */
+    function example_civicrm_alterAdminPanel(&$adminPanel) {
+      // don't want to show Multi Site Settings to users as a configuration option
+      unset($adminPanel['System_Settings']['fields']['weight}.Multi Site Settings']);
+    }