docs: Add some notes about validation that are performed

docs/api/core.rst

@@ -13,6 +13,12 @@ frontends and the backends. Don't forget that you will be accessing core
 as a Pykka actor. If you are only interested in being notified about changes
 in core see :class:`~mopidy.core.CoreListener`.
 
+.. versionchanged:: 1.1
+    All core API calls are now type checked.
+
+.. versionchanged:: 1.1
+    All backend return values are now type checked.
+
 .. autoclass:: mopidy.core.Core
 
   .. attribute:: tracklist

docs/api/models.rst

@@ -5,7 +5,7 @@
 These immutable data models are used for all data transfer within the Mopidy
 backends and between the backends and the MPD frontend. All fields are optional
 and immutable. In other words, they can only be set through the class
-constructor during instance creation.
+constructor during instance creation. Additionally fields are type checked.
 
 If you want to modify a model, use the
 :meth:`~mopidy.models.ImmutableObject.replace` method. It accepts keyword

docs/extensiondev.rst

@@ -437,6 +437,9 @@ When writing an extension, you should only use APIs documented at
 :ref:`api-ref`. Other parts of Mopidy, like :mod:`mopidy.internal`, may change
 at any time and are not something extensions should use.
 
+Mopidy performs type checking to help catch extension bugs. This applies to
+both to frontend calls into core and return values from backends. Additionally
+model fields always get validated to further guard against bad data.
 
 Logging in extensions
 =====================