avcodec/adpcm: update get_nb_samples() doc

Signed-off-by: Zane van Iperen <zane@zanevaniperen.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>
diff --git a/libavcodec/adpcm.c b/libavcodec/adpcm.c
index 7d35884..9ea6a26 100644
--- a/libavcodec/adpcm.c
+++ b/libavcodec/adpcm.c
@@ -668,7 +668,7 @@
 }
 
 /**
- * Get the number of samples that will be decoded from the packet.
+ * Get the number of samples (per channel) that will be decoded from the packet.
  * In one case, this is actually the maximum number of samples possible to
  * decode with the given buf_size.
  *