Add more known bugs to the USB audio manual page.

MFC after:	1 week

share/man/man4/snd_uaudio.4

@@ -29,7 +29,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd February 24, 2011
+.Dd March 19, 2015
 .Dt SND_UAUDIO 4
 .Os
 .Sh NAME
@@ -91,12 +91,25 @@ by
 The
 .Tn PCM
 framework in
-.Fx ,
-as of this writing, does not handle device un-registrations in a properly
-abstracted manner, i.e., a detach request is refused by the
+.Fx
+only supports synchronous device detach.
+That means all mixer and DSP character devices belonging to a given
+USB audio device must be closed when receiving an error on a DSP read,
+a DSP write or a DSP IOCTL request.
+Else the USB audio driver will wait for this to happen, preventing
+enumeration of new devices on the parenting USB controller.
+.Pp
+Some USB audio devices might refuse to work properly unless the sample
+rate is configured the same for both recording and playback, even if
+only simplex is used. See the
+.Va dev.pcm.%d.[play|rec].vchanrate
+sysctls.
+.Pp
+The
 .Tn PCM
-framework if the device is in use.
-It is necessary to allow the device un-registration to complete
-successfully, otherwise the
-.Tn PCM
-layer will panic.
+framework in
+.Fx
+currently doesn't support the full set of USB audio mixer
+controls. Some mixer controls are only available as
+.Va dev.pcm.%d.mixer
+sysctls.