patchbomb: make diffstat usage clearer

Making it clear that diffstat output does not only depend on diffstat
being installed - it also needs to be explicitely requested.

hgext/patchbomb.py

@@ -189,8 +189,9 b' def patchbomb(ui, repo, *revs, **opts):'
     the first line of the changeset description as the subject text.
     The message contains two or three parts. First, the changeset
     description. Next, (optionally) if the diffstat program is
-    installed, the result of running diffstat on the patch. Finally,
-    the patch itself, as generated by "hg export".
+    installed and -d/--diffstat is used, the result of running
+    diffstat on the patch. Finally, the patch itself, as generated by
+    "hg export".
 
     By default the patch is included as text in the email body for
     easy reviewing. Using the -a/--attach option will instead create