docs: Clarify semantics of sparse storage volumes

Sparse LVM volumes do not behave in the way one would naively expect.
The allocation does not automatically increase (which is different from
how sparse files work).

docs/formatstorage.html.in

@@ -250,7 +250,11 @@
         allocated at time of creation. If set to a value smaller than the
         capacity, the pool has the <strong>option</strong> of deciding
         to sparsely allocate a volume. It does not have to honour requests
-        for sparse allocation though.<br/>
+        for sparse allocation though. Different types of pools may treat
+        sparse volumes differently. For example, the <code>logical</code>
+        pool will not automatically expand volume's allocation when it
+        gets full; the user is responsible for doing that or configuring
+        dmeventd to do so automatically.<br/>
         <br/>
         By default this is specified in bytes, but an optional attribute
         <code>unit</code> can be specified to adjust the passed value.