Fix grammar and document get_current_users_in_room (#4998)

changelog.d/4998.misc

synapse/handlers/directory.py

synapse/handlers/events.py

synapse/handlers/message.py

synapse/handlers/presence.py

synapse/handlers/room_list.py

synapse/handlers/sync.py

synapse/handlers/typing.py

synapse/handlers/user_directory.py

synapse/rest/client/v1/admin.py

synapse/state/__init__.py

synapse/storage/user_directory.py

tests/handlers/test_typing.py