token-2022: Clarify when transfer hooks are not called (#28)

#### Problem

It's not clear that the transfer hook is not called during
self-transfers.

#### Summary of changes

Make it abundantly clear.

Author: joncinque

docs/content/docs/token-2022/extensions.mdx

@@ -1536,6 +1536,9 @@ the program may require are automatically resolved on-chain! This process is
 explained in detail in many of the linked `README` files below under
 **Resources**.
 
+**Note**: The transfer-hook program is not called during self-transfers, where
+the source and destination accounts are the same.
+
 #### Resources
 
 The interface description and structs exist at