MDL-50313 docs: Fix the documentation for the FEATURE_IDNUMBER constant

The constant is used by course/moodleform_mod.php to decide whether the
field 'ID number' (cmidnumber) should be included in the common module
settings section of the activity settings form. Activity modules that do
not use grades (such as the Label) can explicitly return false when
checking for this feature support. By default, all activity modules are
supposed to support it.

lib/moodlelib.php

@@ -408,7 +408,7 @@ define('FEATURE_COMPLETION_HAS_RULES', 'completion_has_rules');
 
 /** True if module has no 'view' page (like label) */
 define('FEATURE_NO_VIEW_LINK', 'viewlink');
-/** True if module supports outcomes */
+/** True (which is default) if the module wants support for setting the ID number for grade calculation purposes. */
 define('FEATURE_IDNUMBER', 'idnumber');
 /** True if module supports groups */
 define('FEATURE_GROUPS', 'groups');