Slightly more documentation comment.

cid.go

@@ -409,12 +409,19 @@ func (c Cid) Bytes() []byte {
 	return []byte(c.str)
 }
 
-// ByteLen returns len(c.Bytes()) without an allocation
+// ByteLen returns the length of the CID in bytes.
+// It's equivalent to `len(c.Bytes())`, but works without an allocation,
+// and should therefore be preferred.
+//
+// (See also the WriteTo method for other important operations that work without allocation.)
 func (c Cid) ByteLen() int {
 	return len(c.str)
 }
 
-// WriteTo writes the cids bytes to the given writer
+// WriteTo writes the CID bytes to the given writer.
+// This method works without incurring any allocation.
+//
+// (See also the ByteLen method for other important operations that work without allocation.)
 func (c Cid) WriteTo(w io.Writer) (int, error) {
 	return io.WriteString(w, c.str)
 }