Fix "duplicate object description" error building docs

Warning, treated as error:

/home/travis/build/Supervisor/supervisor/.tox/docs/lib/python3.7/site-packages/supervisor/rpcinterface.py:docstring of supervisor.rpcinterface.SupervisorNamespaceRPCInterface:1:duplicate object description of supervisor.rpcinterface.SupervisorNamespaceRPCInterface, other instance in api, use :noindex: for one of them

Makefile:29: recipe for target 'html' failed

make: *** [html] Error 2

make: Leaving directory '/home/travis/build/Supervisor/supervisor/docs'

ERROR: InvocationError for command /usr/bin/make -C docs html BUILDDIR=/home/travis/build/Supervisor/supervisor/.tox/docs/tmp 'SPHINXOPTS=-W -E' (exited with code 2)
___________________________________ summary ____________________________________

ERROR:   docs: commands failed

Author: mnaberez

docs/api.rst

@@ -225,6 +225,7 @@ Process Control
 ---------------
 
   .. autoclass:: SupervisorNamespaceRPCInterface
+    :noindex:
 
     .. automethod:: getProcessInfo
 
@@ -352,6 +353,7 @@ Process Logging
 ---------------
 
   .. autoclass:: SupervisorNamespaceRPCInterface
+    :noindex:
 
     .. automethod:: readProcessStdoutLog