docs: add more documentation of WLAN configuration

Fixes #574

docs/features/wlan-configuration.rst

+WLAN configuration
+==================
+
+Gluon allows to configure 2.4GHz and 5GHz radios independently. The configuration
+may include any or all of the three networks "client" (AP mode), "mesh" (802.11s
+mode) and "ibss" (adhoc mode), which can be used simultaneously (using "mesh" and
+"ibss" at same time should be avoided though as weaker hardware usually can't handle the additional
+load). See :doc:`../user/site` for details on the configuration.
+
+Upgrade behaviour
+-----------------
+
+For each of these networks, the site configuration may define a `disabled` flag (by
+default, all configured networks are enabled). This flag is merely a default setting,
+on upgrades the existing setting is always retained (as this setting may have been changed
+by the user). This means that is is not possible to enable or disable an existing network
+configurations during upgrades.
+
+For the "mesh" and "ibss" networks, the default setting only has an effect if none
+of the two has existed before. If a new configuration has been added for "mesh" or "ibss",
+while the other of the two has already existed before, the enabled/disabled state of the
+existing configuration will also be set for the new configuration.
+
+This allows upgrades to change from IBSS to 11s and vice-versa while retaining the
+"wireless meshing is enabled/disabled" property configured by the user regardless
+of the used mode.

docs/index.rst

@@ -24,6 +24,7 @@ Features
 
    features/configmode
    features/autoupdater
+   features/wlan-configuration
    features/private-wlan
    features/wired-mesh
    features/announce