MDL-48660 Availability: filter_user_list() should respect 'view hidden'

Updated filter_user_list to account for the viewhiddenactivities and
viewhiddensections capabilities.

availability/classes/tree_node.php

@@ -139,7 +139,8 @@ abstract class tree_node {
 
     /**
      * Tests this condition against a user list. Users who do not meet the
-     * condition will be removed from the list.
+     * condition will be removed from the list, unless they have the ability
+     * to view hidden activities/sections.
      *
      * This function must be implemented if is_applied_to_user_lists returns
      * true. Otherwise it will not be called.
@@ -150,6 +151,10 @@ abstract class tree_node {
      * Within this function, if you need to check capabilities, please use
      * the provided checker which caches results where possible.
      *
+     * Conditions do not need to check the viewhiddenactivities or
+     * viewhiddensections capabilities. These are handled by
+     * core_availability\info::filter_user_list.
+     *
      * @param array $users Array of userid => object
      * @param bool $not True if this condition is applying in negative mode
      * @param \core_availability\info $info Item we're checking