python: supply docstrings for the most used pluginmanager APIs.

It's fun to test out docstring and type annotation use in an IDE only to
hit an undocumented function as the very first thing. Sigh.

src/python/corrade/pluginmanager.h

@@ -84,7 +84,7 @@ template<class T> void manager(py::class_<PluginManager::Manager<T>, PluginManag
             }
 
             return PluginManager::pyPluginHolder(std::move(loaded), py::cast(self));
-        })
+        }, "Instantiate a plugin")
         .def("load_and_instantiate", [](PluginManager::Manager<T>& self, const std::string& plugin) {
             auto loaded = self.loadAndInstantiate(plugin);
             if(!loaded) {
@@ -93,7 +93,7 @@ template<class T> void manager(py::class_<PluginManager::Manager<T>, PluginManag
             }
 
             return PluginManager::pyPluginHolder(std::move(loaded), py::cast(self));
-        });
+        }, "Load and instantiate plugin");
 }
 
 }