MDL-37518 blocks: described the introduction of the myaddinstance capability in upgrade.txt

blocks/upgrade.txt

 This files describes API changes in /blocks/* - activity modules,
 information provided here is intended especially for developers.
 
+=== 2.4 ===
+
+Created new capability 'blocks/xxx:myaddinstance' that determines whether a user can add
+a specific block to their My Home page. This capability was only defined for blocks where
+the applicable_formats function does not include "'my' => false" in the returned array,
+allowing it be added to the My Home page.
+
 === 2.3 ===
 
 required changes in code: