doc/reference: document `FixtureLookupError`

doc/en/example/simple.rst

@@ -168,7 +168,7 @@ Now we'll get feedback on a bad argument:
 
 
 If you need to provide more detailed error messages, you can use the
-``type`` parameter and raise ``pytest.UsageError``:
+``type`` parameter and raise :exc:`pytest.UsageError`:
 
 .. code-block:: python
 

doc/en/reference/reference.rst

@@ -1157,7 +1157,10 @@ When set (regardless of value), pytest will use color in terminal output.
 Exceptions
 ----------
 
-.. autoclass:: pytest.UsageError()
+.. autoexception:: pytest.UsageError()
+    :show-inheritance:
+
+.. autoexception:: pytest.FixtureLookupError()
     :show-inheritance:
 
 .. _`warnings ref`: