qapi/audio: add documentation for AudioFormat

The review for patch ed2a4a7941 "audio: proper support for
float samples in mixeng" suggested this would be a good idea.

Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Volker Rümelin <vr_qemu@t-online.de>
Tested-by: John Arbuckle <programmingkidx@gmail.com>
Message-id: 20200308193321.20668-1-vr_qemu@t-online.de
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>

qapi/audio.json

@@ -273,6 +273,20 @@
 #
 # An enumeration of possible audio formats.
 #
+# @u8: unsigned 8 bit integer
+#
+# @s8: signed 8 bit integer
+#
+# @u16: unsigned 16 bit integer
+#
+# @s16: signed 16 bit integer
+#
+# @u32: unsigned 32 bit integer
+#
+# @s32: signed 32 bit integer
+#
+# @f32: single precision floating-point (since 5.0)
+#
 # Since: 4.0
 ##
 { 'enum': 'AudioFormat',