docs: Add warning to preloading section.

docs/howitworks.rst

@@ -604,6 +604,11 @@ Import Preloading
 
 .. currentmodule:: mitogen.core
 
+.. warning::
+
+    Preloading is not yet implemented: modules are presently still requested
+    one per round-trip.
+
 To further avoid round-trips, when a module or package is requested by a child,
 its bytecode is scanned in the master to find all the module's
 :keyword:`import` statements, and of those, which associated modules appear to