Clarify bot's E2E encryption caveat

Update the README to specify that issues with file attachments primarily occur in non-encrypted rooms when the same user operates the bot in both encrypted and non-encrypted rooms. This detail aims to inform users of potential pitfalls more precisely when setting up the bot with end-to-end encryption enabled.

README.md

@@ -51,7 +51,8 @@ to install the latest version from the Git repository.
 #### End-to-end encryption
 
 WARNING: Using end-to-end encryption seems to sometimes cause problems with
-file attachments, even in rooms that are not encrypted.
+file attachments, especially in rooms that are not encrypted, if the same
+user also uses the bot in encrypted rooms.
 
 The bot itself does not implement end-to-end encryption. However, it can be
 used in conjunction with [pantalaimon](https://github.com/matrix-org/pantalaimon),