doc: Document station Affinities property

This documents new DBus property that expose a bit more control to
how IWD roams.

Setting the affinity on the connected BSS effectively "locks" IWD to
that BSS (except at critical RSSI levels, explained below). This can
be useful for clients that have access to more information about the
environment than IWD. For example, if a client is stationary there
is likely no point in trying to roam until it has moved elsewhere.

A new main.conf option would also be added:

[General].CriticalRoamThreshold

This would be the new roam threshold set if the currently connected
BSS is in the Affinities list. If the RSSI continues to drop below
this level IWD will still attempt to roam.

doc/station-api.txt

@@ -170,6 +170,23 @@ Properties	string State [readonly]
 			BSS the device is currently connected to or to which
 			a connection is in progress.
 
+		ao Affinities [optional] [experimental]
+
+			Array of net.connman.iwd.BasicServiceSet object paths
+			that will be treated with higher affinity compared to
+			other BSS's. Currently the only allowed value to be
+			set in this array is the path to the currently connected
+			BasicServiceSet object, i.e.
+			Station.ConnectedAccessPoint.
+
+			Setting the affinity will lower the roaming threshold,
+			effectively locking IWD to the current BSS unless the
+			RSSI drops below the critical threshold set by
+			[General].CriticalRoamThreshold{5G} at which point
+			IWD will proceed with normal roaming behavior.
+
+			This property is cleared on roams/disconnections.
+
 SignalLevelAgent hierarchy
 ==========================
 

src/iwd.config.rst

@@ -133,6 +133,22 @@ The group ``[General]`` contains general settings.
        This value can be used to control how aggressively **iwd** roams when
        connected to a 5GHz access point.
 
+   * - CriticalRoamThreshold
+     - Value: rssi dBm value, from -100 to -1, default: **-80**
+
+       The threshold (for 2.4GHz) at which IWD will roam regardless of the
+       affinity set to the current BSS. If the connected BSS has affinity
+       (set in Station's Affinities list) the roam threshold will be lowed to
+       this value and IWD will not attempt to roam (or roam scan) until either
+       the affinity is cleared, or the signal drops below this threshold.
+
+
+   * - CriticalRoamThreshold5G
+     - Value: rssi dBm value, from -100 to -1, default: **-82**
+
+       This has the same effect as ``CriticalRoamThreshold``, but for the 5GHz
+       band.
+
    * - RoamRetryInterval
      - Value: unsigned int value in seconds (default: **60**)