Document BEAM action

Button-functions.md

@@ -125,6 +125,7 @@ Available functions:
 * VOX - turn voice activation function ON / OFF
 * FM RADIO - turn FM radio ON / OFF
 * `1750 Hz` - send the `1750 Hz` tone burst
+* BEAM - opens BEAM transfer mode, when enabled in the build. BEAM can send the current VFO/memory-channel settings to another radio or receive settings from another radio.
 * LOCK KEYPAD - lock / unlock the keypad
 * VFO A VFO B - change main VFO to upper/lower
 * VFO MEM - change current VFO mode, frequency mode or memory channel mode
@@ -138,6 +139,20 @@ Available functions:
 * POWER HIGH - temporarily switch to the maximum power of `5 W`
 * REMOVE OFFSET - temporarily remove the offset of a memory channel, if present
 
+### BEAM action
+
+BEAM is available only in builds compiled with BEAM support. Assign `BEAM` to one of the customizable shortcuts (`F1Shrt`, `F1Long`, `F2Shrt`, `F2Long`, or `M Long`), then trigger that shortcut to open BEAM mode.
+
+In BEAM mode:
+
+* `UP` / `DOWN` toggles between `BEAM TX` and `BEAM RX`
+* `M` starts the selected operation
+* `EXIT` leaves BEAM mode
+
+`BEAM TX` sends the current VFO or memory-channel configuration. The packet includes the RX frequency, TX offset, RX/TX DCS or CTCSS settings, modulation, bandwidth, output power, scan-list assignment, compander, scrambling setting, DTMF-related settings when enabled, and the channel name.
+
+`BEAM RX` waits for a BEAM packet from another radio. When a valid packet is received, the radio saves it to the first free memory channel. If memory is full, the status shows `MEM FULL`. Pressing `EXIT` after a successful receive switches to the newly saved channel; otherwise it restores the previous VFO/channel state.
+
 ## Related pages
 
 * [Getting started](./Getting-started)