doc: Elaborate on instance volume sizes

Signed-off-by: hamistao <[email protected]>

doc/howto/storage_volumes.md

@@ -196,5 +196,5 @@ To resize a storage volume, set its size configuration:
 - Shrinking a storage volume is only possible for storage volumes with content type `filesystem`.
   It is not guaranteed to work though, because you cannot shrink storage below its current used size.
 - Shrinking a storage volume with content type `block` is not possible.
-
+- Directly resizing a volume is only allowed for custom volumes. Although it can't be manually set, the `size` property on `container` and `virtual-machine` volumes contains the instance's size at any given point in time.
 ```