device-manager: Add a comment about non-extensible struct

This comment can potentially save a lot of debugging effort and fixing
an ABI break, even though I don't think it's particularly likely that
anyone will ever extend pa_ext_device_manager_role_priority_info.

src/pulse/ext-device-manager.h

@@ -34,6 +34,9 @@
 
 PA_C_DECL_BEGIN
 
+/* Don't extend this struct! It will break binary compatibility, because
+ * pa_ext_device_manager_info.role_priorities points to an array of structs
+ * instead of an array of pointers to structs. */
 typedef struct pa_ext_device_manager_role_priority_info {
     const char *role;
     uint32_t priority;