RTL-SDR: updated documentation

plugins/samplesource/rtlsdr/readme.md

@@ -106,7 +106,11 @@ The I/Q stream from the RTLSDR ADC is doensampled by a power of two before being
 
 Use this checkbox to activate the special RTLSDR direct sampling. This can be used to tune to HF frequencies.
 
-<h3>9: RF gain and AGC</h2>
+<h3>9: RF bandwidth</h3>
+
+This controls the tuner filter bandwidth and can be varied from 350 kHz to 8 MHz. In practice depending on the value this appears to be larger and the filter center is slightly offset above the center frequency. This can still be very useful to eliminate or attenuate large signals outside the device to host I/Q stream passband.
+
+<h3>10: RF gain and AGC</h2>
 
 The slider sets RF gain in dB. The values are defined in the RTLSDR device and generally are: 0.0, 0.9, 1.4, 2.7, 3.7, 7.7, 8.7, 12.5, 14.4, 15.7, 16.6, 19.7, 20.7, 22.9, 25.4, 28.0, 29.7, 32.8, 33.8, 36.4, 37.2, 38.6, 40.2, 42.1, 43.4, 43.9, 44.5, 48.0, 49.6