tiff/lzw: sync to the upstream lzw in the stdlib.

Change-Id: If6086ada81498327c61e91b18d135aa255401e68
Reviewed-on: https://go-review.googlesource.com/11320
Reviewed-by: David Symonds <dsymonds@golang.org>

tiff/lzw/reader.go

@@ -237,10 +237,13 @@ func (d *decoder) Close() error {
 
 // NewReader creates a new io.ReadCloser.
 // Reads from the returned io.ReadCloser read and decompress data from r.
+// If r does not also implement io.ByteReader,
+// the decompressor may read more data than necessary from r.
 // It is the caller's responsibility to call Close on the ReadCloser when
 // finished reading.
 // The number of bits to use for literal codes, litWidth, must be in the
-// range [2,8] and is typically 8.
+// range [2,8] and is typically 8. It must equal the litWidth
+// used during compression.
 func NewReader(r io.Reader, order Order, litWidth int) io.ReadCloser {
 	d := new(decoder)
 	switch order {