Make the definition of buffer length more explicit

This is the standard definition, but apparently it's still confusing.
Resolves #47.

src/buffer.rs

@@ -28,7 +28,7 @@ pub struct Buffer<'a> {
 }
 
 impl<'a> Buffer<'a> {
-    /// Returns the number of samples in this buffer.
+    /// Returns the number of samples per channel in this buffer.
     #[inline]
     pub fn len(&self) -> usize {
         if self.output_slices.is_empty() {

src/buffer/blocks.rs

@@ -124,7 +124,7 @@ impl ExactSizeIterator for BlocksIter<'_, '_> {}
 impl ExactSizeIterator for BlockChannelsIter<'_, '_> {}
 
 impl<'slice, 'sample> Block<'slice, 'sample> {
-    /// Get the number of samples (not channels) in the block.
+    /// Get the number of samples per channel in the block.
     #[allow(clippy::len_without_is_empty)]
     #[inline]
     pub fn len(&self) -> usize {