Document the -Y filter option (closes #3044)

The FM low-pass filter cutoff for the FSK demodulator was implemented
but never listed in -h/README, despite being the fix recommended in
issue #1811 for separating simultaneous transmissions.

README.md

@@ -68,6 +68,7 @@ See [CONTRIBUTING.md](./docs/CONTRIBUTING.md).
   [-Y autolevel] Set minlevel automatically based on average estimated noise.
   [-Y squelch] Skip frames below estimated noise level to reduce cpu load.
   [-Y ampest | magest] Choose amplitude or magnitude level estimator.
+  [-Y filter=<value>] Manual FM low-pass filter cutoff to separate simultaneous transmissions: us (1-9999, e.g. 20), Hz (10000+), or ratio of sample rate (0.0-1.0).
 		= Analyze/Debug options =
   [-A] Pulse Analyzer. Enable pulse analysis and decode attempt.
        Disable all decoders with -R 0 if you want analyzer output only.

man/man1/rtl_433.1

@@ -105,6 +105,9 @@ Skip frames below estimated noise level to reduce cpu load.
 .TP
 [ \fB\-Y\fI ampest | magest\fP ]
 Choose amplitude or magnitude level estimator.
+.TP
+[ \fB\-Y\fI filter=<value>\fP ]
+Manual FM low\-pass filter cutoff to separate simultaneous transmissions: us (1\-9999, e.g. 20), Hz (10000+), or ratio of sample rate (0.0\-1.0).
 .SS "Analyze/Debug options"
 .TP
 [ \fB\-A\fI\fP ]

src/rtl_433.c

@@ -141,6 +141,7 @@ static void usage(int exit_code)
             "  [-Y autolevel] Set minlevel automatically based on average estimated noise.\n"
             "  [-Y squelch] Skip frames below estimated noise level to reduce cpu load.\n"
             "  [-Y ampest | magest] Choose amplitude or magnitude level estimator.\n"
+            "  [-Y filter=<value>] Manual FM low-pass filter cutoff to separate simultaneous transmissions: us (1-9999, e.g. 20), Hz (10000+), or ratio of sample rate (0.0-1.0).\n"
             "\t\t= Analyze/Debug options =\n"
             "  [-A] Pulse Analyzer. Enable pulse analysis and decode attempt.\n"
             "       Disable all decoders with -R 0 if you want analyzer output only.\n"