Update docs with more howto examples (#968)

Co-authored-by: Teemu R. <tpr@iki.fi>

kasa/smart/modules/childdevice.py

@@ -1,4 +1,42 @@
-"""Implementation for child devices."""
+"""Interact with child devices.
+
+>>> from kasa import Discover
+>>>
+>>> dev = await Discover.discover_single(
+>>>     "127.0.0.1",
+>>>     username="user@example.com",
+>>>     password="great_password"
+>>> )
+>>> await dev.update()
+>>> print(dev.alias)
+Bedroom Power Strip
+
+All methods act on the whole strip:
+
+>>> for plug in dev.children:
+>>>    print(f"{plug.alias}: {plug.is_on}")
+Plug 1: True
+Plug 2: False
+Plug 3: False
+>>> dev.is_on
+True
+>>> await dev.turn_off()
+>>> await dev.update()
+
+Accessing individual plugs can be done using the `children` property:
+
+>>> len(dev.children)
+3
+>>> for plug in dev.children:
+>>>    print(f"{plug.alias}: {plug.is_on}")
+Plug 1: False
+Plug 2: False
+Plug 3: False
+>>> await dev.children[1].turn_on()
+>>> await dev.update()
+>>> dev.is_on
+True
+"""
 
 from ..smartmodule import SmartModule