storage/callback: remove the "word of caution"

As discussed in the PR, sync code will not be interrupted when run from
async code as long as Qubes OS doesn't run dedicated threads for async
& sync code. So there's simply no issue to be expected and thus no special
caution required.

qubes/storage/callback.py

@@ -46,12 +46,6 @@ class CallbackPool(qubes.storage.Pool):
         - custom pool mounts
         - encryption
         - debugging
-        - run synchronous pool drivers asynchronously
-
-    A word of caution:
-    This implementation runs all methods that `qubes.storage.Pool` allows to be asynchronous asynchronously. So if a backend pool driver does
-    not support a particular method to be run asynchronously, there may be issues. In short, it is always preferable to use the original backend
-    driver over this one unless the functionality of this driver is required for a particular use case.
 
 
     **Integration tests**: