compress/lzw: mention relation between litWidth and input bytes

Add sentences to the docs explaining the limit on input
bytes implicit in the choice of litWidth, and the fact that
compress and decompress litWidth must match.

Fixes #11142.

Change-Id: I20cfb4df35739f7bfeb50b92c78249df3d47942c
Reviewed-on: https://go-review.googlesource.com/11063Reviewed-by: Nigel Tao <nigeltao@golang.org>

src/compress/lzw/reader.go

@@ -227,7 +227,8 @@ func (d *decoder) Close() error {
 // 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 {

src/compress/lzw/writer.go

@@ -230,7 +230,7 @@ func (e *encoder) Close() error {
 // It is the caller's responsibility to call Close on the WriteCloser when
 // finished writing.
 // 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. Input bytes must be less than 1<<litWidth.
 func NewWriter(w io.Writer, order Order, litWidth int) io.WriteCloser {
 	var write func(*encoder, uint32) error
 	switch order {