Document which type are nullable, and wire format for null value

Signed-off-by: Ian Douglas Scott <idscott@system76.com>

doc/publican/sources/Protocol.xml

@@ -151,7 +151,8 @@
 	    <para>
 	      Starts with an unsigned 32-bit length (including null terminator),
 	      followed by the string contents, including terminating null byte,
-	      then padding to a 32-bit boundary.
+	      then padding to a 32-bit boundary. A null value is represented
+	      with a length of 0.
 	    </para>
 	  </listitem>
 	</varlistentry>
@@ -159,7 +160,7 @@
 	  <term>object</term>
 	  <listitem>
 	    <para>
-	      32-bit object ID.
+	      32-bit object ID. A null value is represented with an ID of 0.
 	    </para>
 	  </listitem>
 	</varlistentry>

src/wayland-util.h

@@ -118,7 +118,7 @@ struct wl_object;
  * * `n`: new_id
  * * `a`: array
  * * `h`: fd
- * * `?`: following argument is nullable
+ * * `?`: following argument (`o` or `s`) is nullable
  *
  * While demarshaling primitive arguments is straightforward, when demarshaling
  * messages containing `object` or `new_id` arguments, the protocol