compress: update documentation regarding footer verification

Address two documentation issues:
1) Document that the GZIP and ZLIB footer is only verified when the
reader has been fully consumed.
2) The zlib reader is guaranteed to not read past the EOF if the
input io.Reader is also a io.ByteReader. This functionality was
documented in the flate and gzip packages but not on zlib.

Fixes #14867

Change-Id: I43d46b93e38f98a04901dc7d4f18ed2f9e09f6fb
Reviewed-on: https://go-review.googlesource.com/21218Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/compress/gzip/gunzip.go

@@ -282,4 +282,6 @@ func (z *Reader) Read(p []byte) (n int, err error) {
 }
 
 // Close closes the Reader. It does not close the underlying io.Reader.
+// In order for the GZIP checksum to be verified, the reader must be
+// fully consumed until the io.EOF.
 func (z *Reader) Close() error { return z.decompressor.Close() }

src/compress/zlib/reader.go

@@ -62,7 +62,8 @@ type Resetter interface {
 
 // NewReader creates a new ReadCloser.
 // Reads from the returned ReadCloser read and decompress data from r.
-// The implementation buffers input and may read more data than necessary from r.
+// If r does not 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 done.
 //
 // The ReadCloser returned by NewReader also implements Resetter.
@@ -115,6 +116,8 @@ func (z *reader) Read(p []byte) (int, error) {
 }
 
 // Calling Close does not close the wrapped io.Reader originally passed to NewReader.
+// In order for the ZLIB checksum to be verified, the reader must be
+// fully consumed until the io.EOF.
 func (z *reader) Close() error {
 	if z.err != nil && z.err != io.EOF {
 		return z.err