fix Truncate comment:

* make a complete sentence.
* eliminate reference to byte positions,
  which are not a concept exposed by the interface.

R=gri
DELTA=2  (0 added, 1 deleted, 1 changed)
OCL=28838
CL=28838

src/lib/io/bytebuffer.go

@@ -39,8 +39,7 @@ func (b *ByteBuffer) Len() int {
 	return len(b.buf) - b.off
 }
 
-// Truncates the buffer so it contains n bytes.
-// It preserves the data in the buffer at positions [0 : n].
+// Truncate discards all but the first n unread bytes from the buffer.
 // It is an error to call b.Truncate(n) with n > b.Len().
 func (b *ByteBuffer) Truncate(n int) {
 	b.buf = b.buf[0 : b.off + n];