compress/*: note that NewReader may introduce buffering

Fixes #8309.

LGTM=r
R=golang-codereviews, r
CC=golang-codereviews, iant
https://golang.org/cl/147380043

src/compress/bzip2/bzip2.go

@@ -42,6 +42,8 @@ type reader struct {
 }
 
 // NewReader returns an io.Reader which decompresses bzip2 data from r.
+// If r does not also implement io.ByteReader,
+// the decompressor may read more data than necessary from r.
 func NewReader(r io.Reader) io.Reader {
 	bz2 := new(reader)
 	bz2.br = newBitReader(r)

src/compress/flate/inflate.go

@@ -680,9 +680,11 @@ func makeReader(r io.Reader) Reader {
 }
 
 // NewReader returns a new ReadCloser that can be used
-// to read the uncompressed version of r.  It is the caller's
+// to read the uncompressed version of r.
-// responsibility to call Close on the ReadCloser when
+// If r does not also implement io.ByteReader,
-// finished reading.
+// 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.
 func NewReader(r io.Reader) io.ReadCloser {
 	var f decompressor
 	f.bits = new([maxLit + maxDist]int)

src/compress/gzip/gunzip.go

@@ -77,7 +77,8 @@ type Reader struct {
 }
 
 // NewReader creates a new Reader reading the given reader.
-// The implementation buffers input and may read more data than necessary 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 Reader when done.
 func NewReader(r io.Reader) (*Reader, error) {
 	z := new(Reader)

src/compress/lzw/reader.go

@@ -222,6 +222,8 @@ 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