scripts/clean-includes: Improve --git commit message

The script drops #include "qemu/osdep.h" from headers.  Mention it in
the commit message it uses for --git.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Message-Id: <20230202133830.2152150-5-armbru@redhat.com>

scripts/clean-includes

@@ -193,11 +193,17 @@ if [ "$GIT" = "yes" ]; then
     git commit --signoff -F - <<EOF
 $GITSUBJ: Clean up includes
 
-Clean up includes so that osdep.h is included first and headers
-which it implies are not included manually.
-
 This commit was created with scripts/clean-includes.
 
+All .c should include qemu/osdep.h first.  The script performs three
+related cleanups:
+
+* Ensure .c files include qemu/osdep.h first.
+* Including it in a .h is redundant, since the .c  already includes
+  it.  Drop such inclusions.
+* Likewise, including headers qemu/osdep.h includes is redundant.
+  Drop these, too.
+
 EOF
 
 fi