MDL-59502 Global Search: deprecate add_documents 4 arg implementation

In MDL-59039 we introduced changes to add_documents and should return an extra $partial boolean.
We still supported implementations returning 4 elements since then,
but  this issue is about removing this 4 returned elements compatibility.

search/classes/manager.php

@@ -1120,11 +1120,7 @@ class manager {
             if (count($result) === 5) {
                 list($numrecords, $numdocs, $numdocsignored, $lastindexeddoc, $partial) = $result;
             } else {
-                // Backward compatibility for engines that don't support partial adding.
-                list($numrecords, $numdocs, $numdocsignored, $lastindexeddoc) = $result;
-                debugging('engine::add_documents() should return $partial (4-value return is deprecated)',
-                        DEBUG_DEVELOPER);
-                $partial = false;
+                throw new coding_exception('engine::add_documents() should return $partial (4-value return is deprecated)');
             }
 
             if ($numdocs > 0) {

search/upgrade.txt

@@ -1,6 +1,13 @@
 This files describes API changes in /search/*,
 information provided here is intended especially for developers.
 
+=== 3.8 ===
+
+* Search indexing supports time limits to make the scheduled task run more neatly since 3.4. In order for
+  this to work, search engine plugins will need to implement the 'stopat' parameter if they
+  override the add_documents() function, and return an extra parameter from this function (see base
+  class in engine.php). Unmodified plugins will not work anymore.
+
 === 3.7 ===
 
 * Search areas now have categories and can optionally implement get_category_names method to