docs: Add some details

Mention immutability in the object model section.

docs/usermanual-object-model.xml

@@ -69,6 +69,9 @@
       After construction, each object's properties are accessible only
       through the setter and getter functions described in the API
       Reference manual.
+    </para>
+      Note that many object types can be marked as read-only or immutable,
+      facilitating their use in multi-threaded environments.
     </para>
     <para>
       Key object types provided by HarfBuzz include:
@@ -253,6 +256,10 @@
       about freeing memory and let the reference-counting machinery
       take care of that.
     </para>
+    <para>
+      Most of the time, blobs are read-only, facilitating their use in
+      immutable objects.
+    </para>
   </section>
 
 </chapter>