bufio: document Reader must be created with New functions

Fixes #37347

Change-Id: Id642dbe7e52933f8d6cf7116c3c27ddb74efba9d
Reviewed-on: https://go-review.googlesource.com/c/go/+/633315
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Michael Knyszek <[email protected]>

Author: seankhliao

src/bufio/bufio.go

@@ -29,6 +29,9 @@ var (
 // Buffered input.
 
 // Reader implements buffering for an io.Reader object.
+// A new Reader is created by calling [NewReader] or [NewReaderSize];
+// alternatively the zero value of a Reader may be used after calling [Reset]
+// on it.
 type Reader struct {
 	buf          []byte
 	rd           io.Reader // reader provided by the client