docs: Make extensiondev example and API docs for validate_environment() match

(cherry picked from commit 31c874b3eb)

docs/extensiondev.rst

@@ -317,7 +317,7 @@ This is ``mopidy_soundspot/__init__.py``::
             gobject.type_register(SoundspotMixer)
             gst.element_register(
                 SoundspotMixer, 'soundspotmixer', gst.RANK_MARGINAL)
-            
+
             # Or nothing to register e.g. command extension
             pass
 

mopidy/ext.py

@@ -52,7 +52,7 @@ class Extension(object):
         return schema
 
     def get_command(self):
-        """Command to expose to command line users running mopidy.
+        """Command to expose to command line users running ``mopidy``.
 
         :returns:
           Instance of a :class:`~mopidy.commands.Command` class.
@@ -60,12 +60,13 @@ class Extension(object):
         pass
 
     def validate_environment(self):
-        """Checks if the extension can run in the current environment
+        """Checks if the extension can run in the current environment.
 
-        For example, this method can be used to check if all dependencies that
-        are needed are installed. If a problem is found, raise
-        :exc:`~mopidy.exceptions.ExtensionError` with a message explaining the
-        issue.
+        Dependencies described by :file:`setup.py` are checked by Mopidy, so
+        you should not check their presence here.
+
+        If a problem is found, raise :exc:`~mopidy.exceptions.ExtensionError`
+        with a message explaining the issue.
 
         :raises: :exc:`~mopidy.exceptions.ExtensionError`
         :returns: :class:`None`