tests/multi_espnow: Add channel setting test, add some docs.

Test currently passes. It was added so it can be used to check for
regressions when fixing channel selection for AP mode in a follow-up
commit.

Also add some docs about how channel setting is observed to work for
ESP-NOW.

This work was funded through GitHub Sponsors.

Signed-off-by: Angus Gratton <angus@redyak.com.au>

docs/library/espnow.rst

@@ -441,7 +441,9 @@ must first register the sender and use the same encryption keys as the sender
 
         - *channel*: The wifi channel (2.4GHz) to communicate with this peer.
           Must be an integer from 0 to 14. If channel is set to 0 the current
-          channel of the wifi device will be used. (default=0)
+          channel of the wifi device will be used, if channel is set to another
+          value then this must match the channel currently configured on the
+          interface (see :func:`WLAN.config`). (default=0)
 
         - *ifidx*: (ESP32 only) Index of the wifi interface which will be
           used to send data to this peer. Must be an integer set to
@@ -470,6 +472,9 @@ must first register the sender and use the same encryption keys as the sender
           registered.
         - ``OSError(num, "ESP_ERR_ESPNOW_FULL")`` if too many peers are
           already registered.
+        - ``OSError(num, "ESP_ERR_ESPNOW_CHAN")`` if a channel value was
+          set that doesn't match the channel currently configured for this
+          interface.
         - ``ValueError()`` on invalid keyword args or values.
 
 .. method:: ESPNow.del_peer(mac)

docs/library/network.WLAN.rst

@@ -126,7 +126,7 @@ Methods
    =============  ===========
    mac            MAC address (bytes)
    ssid           WiFi access point name (string)
-   channel        WiFi channel (integer)
+   channel        WiFi channel (integer). Depending on the port this may only be supported on the AP interface.
    hidden         Whether SSID is hidden (boolean)
    security       Security protocol supported (enumeration, see module constants)
    key            Access key (string)