bufio: document that NewWriter can return its argument

Fixes #49446

Change-Id: Ib0b53a7dd5d567a2dd0bdf29f53d276587b60afb
Reviewed-on: https://go-review.googlesource.com/c/go/+/361921
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/bufio/bufio.go b/src/bufio/bufio.go
index 063a7785f3..9ea058db3e 100644
--- a/src/bufio/bufio.go
+++ b/src/bufio/bufio.go
@@ -593,6 +593,8 @@ func NewWriterSize(w io.Writer, size int) *Writer {
 }
 
 // NewWriter returns a new Writer whose buffer has the default size.
+// If the argument io.Writer is already a Writer with large enough buffer size,
+// it returns the underlying Writer.
 func NewWriter(w io.Writer) *Writer {
 	return NewWriterSize(w, defaultBufSize)
 }