MDL-72324 core: Add upgrade.txt note about $plugin->incompatible

lib/upgrade.txt

@@ -1,6 +1,12 @@
 This files describes API changes in core libraries and APIs,
 information provided here is intended especially for developers.
 
+=== 3.11.8 ===
+* The check for $plugin->incompatible was found to be incorrect. The $plugin->incompatible attribute is meant to define the minimum
+  Moodle version the plugin is incompatible with but the implemented logic for the check was the opposite. Plugins declaring this
+  attribute may encounter different behaviours between older Moodle versions (<v3.11.8, <v4.0.2) and the later ones. We recommend
+  plugin developers to not use this attribute for Moodle versions 4.0 and below in order to avoid this problem.
+
 === 3.11.6 ===
 * The persistent method from_record() now only attempts to load record properties defined in the persistent class.