MDL-70959 completion: Documentation improvements

* Fix typo in $cm parameter's type in PHPDoc
* Improve the comment to better explain the logic why fetching the
completion data for the whole course can only be done when caching is
used.
Even if $wholecourse is set to true, there's no point in fetching the
completion data for all the course modules unless we're caching the
results.

lib/completionlib.php

@@ -927,7 +927,7 @@ class completion_info {
      * Obtains completion data for a particular activity and user (from the
      * completion cache if available, or by SQL query)
      *
-     * @param stcClass|cm_info $cm Activity; only required field is ->id
+     * @param stdClass|cm_info $cm Activity; only required field is ->id
      * @param bool $wholecourse If true (default false) then, when necessary to
      *   fill the cache, retrieves information from the entire course not just for
      *   this one activity
@@ -965,7 +965,9 @@ class completion_info {
             }
         }
 
-        // Not there, get via SQL
+        // If cached completion data is not found, fetch via SQL.
+        // Fetch completion data for all of the activities in the course ONLY if we're caching the fetched completion data.
+        // If we're not caching the completion data, then just fetch the completion data for the user in this course module.
         if ($usecache && $wholecourse) {
             // Get whole course data for cache
             $alldatabycmc = $DB->get_records_sql("