Add warning on TX config when using Gemini Tx with non-Gemini Rx (#428)

* Add warning on TX config when using Gemini Tx with non-Gemini Rx

* Add reason why use non Gemini Tx mode

docs/quick-start/transmitters/lua-howto.md

@@ -191,6 +191,10 @@ Available Options:
 
     * Alternate between the two Antennas.
 
+!!! warning "WARNING"
+	When using a Gemini TX with a non-Gemini receiver, you should set the TX in either Switch or Single Antenna (Ant1/Ant2) Mode.
+  The reason is that as the receiver only get the signal from the TX it has synchronized with, the other TX will act as a noise generator and increase both the noise floor and power consumption.
+
 ### Link Mode
 
 Introduced in 3.5.0, `Link Mode` changes the main protocol and function of the TX module.