os: clarify that File.{Read,Write} use the buffer

Fixes #49470

Change-Id: I81fd4b0e2eef1d8d430b5d1d10c4f824e803a75c
Reviewed-on: https://go-review.googlesource.com/c/go/+/362335


Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/os/file.go

@@ -109,7 +109,7 @@ func (e *LinkError) Unwrap() error {
 	return e.Err
 }
 
-// Read reads up to len(b) bytes from the File.
+// Read reads up to len(b) bytes from the File and stores them in b.
 // It returns the number of bytes read and any error encountered.
 // At end of file, Read returns 0, io.EOF.
 func (f *File) Read(b []byte) (n int, err error) {
@@ -166,7 +166,7 @@ type onlyWriter struct {
 	io.Writer
 }
 
-// Write writes len(b) bytes to the File.
+// Write writes len(b) bytes from b to the File.
 // It returns the number of bytes written and an error, if any.
 // Write returns a non-nil error when n != len(b).
 func (f *File) Write(b []byte) (n int, err error) {