app/vmauth/main.go: clarify comments for bufferedBody struct a bit

This is a follow-up for https://github.com/VictoriaMetrics/VictoriaMetrics/pull/10677#discussion_r3064731250

app/vmauth/main.go

@@ -357,6 +357,7 @@ func bufferRequestBody(ctx context.Context, r io.ReadCloser, userName string) (i
 
 	maxBufSize := max(requestBufferSize.IntN(), maxRequestBodySizeToRetry.IntN())
 	if maxBufSize <= 0 {
+		// Request buffering is disabled.
 		return r, nil
 	}
 
@@ -792,10 +793,11 @@ func handleConcurrencyLimitError(w http.ResponseWriter, r *http.Request, err err
 }
 
 // bufferedBody serves two purposes:
-//  1. Enables request retries when the body size does not exceed maxBodySize
-//     by fully buffering the body in memory.
-//  2. Prevents slow clients from reducing effective server capacity by
-//     buffering the request body before acquiring a per-user concurrency slot.
+//
+//  1. It enables request retries when the request body size does not exceed maxBufSize
+//     by fully buffering the request body in memory.
+//  2. It prevents slow clients from reducing effective server capacity
+//     by buffering the request body before acquiring a per-user concurrency slot.
 //
 // See bufferRequestBody for details on how bufferedBody is used.
 type bufferedBody struct {
@@ -819,7 +821,7 @@ func newBufferedBody(r io.ReadCloser, buf []byte, maxBufSize int) *bufferedBody
 	// See https://github.com/VictoriaMetrics/VictoriaMetrics/issues/8051
 
 	if len(buf) < maxBufSize {
-		// Read the full request body into buf.
+		// The full request body has been already read into buf.
 		r = nil
 	}
 
@@ -832,7 +834,7 @@ func newBufferedBody(r io.ReadCloser, buf []byte, maxBufSize int) *bufferedBody
 // Read implements io.Reader interface.
 func (bb *bufferedBody) Read(p []byte) (int, error) {
 	if bb.cannotRetry {
-		return 0, fmt.Errorf("cannot read already closed body")
+		return 0, fmt.Errorf("cannot read already closed request body")
 	}
 	if bb.bufOffset < len(bb.buf) {
 		n := copy(p, bb.buf[bb.bufOffset:])