Updated SDRPlay device notes (markdown)

SDRPlay-device-notes.md

@@ -4,7 +4,7 @@ SDRPlay currently provides two major versions of their driver ("API") for Linux
 
 Only the OpenWebRX Raspberry Pi images and the OpenWebRX docker images come with the driver preloaded, if you're installing from packages you will need to install both the driver and the SoapySDRPlay module manually.
 
-# SoapySDR
+# SoapySDR support
 
 There is multiple versions of the SoapySDRPlay module available, depending on the SDRPlay API version the following ones are recommended:
 
@@ -13,4 +13,13 @@ There is multiple versions of the SoapySDRPlay module available, depending on th
 
 # Gain
 
-# RSPDuo: Using both tuners simultaneously
+Like other Soapy supported devices, SDRPlay devices can take a single gain value. However, since the way the underlying gains are set up, this usually results in unpredicted results. See: https://github.com/pothosware/SoapySDRPlay/issues/60
+
+SDRPlay devices export two gain controls, IFGR and RFGR. Both of them are gain reduction values, so the higher the value you set, the lower your signal levels will be. This is important to understand since it is counter-intuitive and pretty much opposite to the behavior of other devices.
+
+RFGR is set in steps starting at 0, the maximum depends on the device and input used. Each step equals approximately 6dB of attenuation.
+IFGR is set in dB, with a minimum of 20.
+
+# RSPDuo: Using both tuners simultaneously
+
+This should be possible using the API version 3, but I haven't been able to get it to work yet.