net/http: add comments for transport gzip reader

Add back comments dropped in CL 510255

Updates golang#61353

Change-Id: Ie84610b830599e178140bb5c2a11be74b2ad461e
GitHub-Last-Rev: d6cd890
GitHub-Pull-Request: golang#75293
Reviewed-on: https://go-review.googlesource.com/c/go/+/701395
Reviewed-by: Sean Liao <[email protected]>
Reviewed-by: Mark Freeman <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>

Author: AlexanderYastrebov

src/net/http/transport.go

@@ -3046,8 +3046,8 @@ type gzipReader struct {
 	_    incomparable
 	body *bodyEOFSignal // underlying HTTP/1 response body framing
 	mu   sync.Mutex     // guards zr and zerr
-	zr   *gzip.Reader
-	zerr error
+	zr   *gzip.Reader   // stores gzip reader from the pool between reads
+	zerr error          // sticky gzip reader init error or sentinel value to detect concurrent read and read after close
 }
 
 type eofReader struct{}