doc/reference: include PluginManager in PytestPluginManager and remove it

Before, `PluginManager` was a copy of the pluggy doc, and
`PytestPluginManager` was documented separately.

pytest users only really need to know about `PytestPluginManager`, so
instead of splitting have the `PytestPluginManager` documentation
include all of `PluginManager` members and remove `PluginManager` from
the reference (it is still shown as the base class).

Author: bluetech

doc/en/reference.rst

@@ -872,19 +872,14 @@ Parser
 .. autoclass:: _pytest.config.argparsing.Parser()
     :members:
 
-PluginManager
-~~~~~~~~~~~~~
-
-.. autoclass:: pluggy.PluginManager()
-    :members:
-
 
 PytestPluginManager
 ~~~~~~~~~~~~~~~~~~~
 
 .. autoclass:: _pytest.config.PytestPluginManager()
     :members:
     :undoc-members:
+    :inherited-members:
     :show-inheritance:
 
 Session