io: document that ReadAtLeast and ReadFull can drop errors

Add a note that if an error is returned after having read
at least the minimum no. of bytes, the error is set to nil.

Fixes #20477

Change-Id: I75ba5ee967be3ff80249e40d459da4afeeb53463
Reviewed-on: https://go-review.googlesource.com/102459Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/io/io.go

@@ -300,6 +300,7 @@ func WriteString(w Writer, s string) (n int, err error) {
 // ReadAtLeast returns ErrUnexpectedEOF.
 // If min is greater than the length of buf, ReadAtLeast returns ErrShortBuffer.
 // On return, n >= min if and only if err == nil.
+// If r returns an error having read at least min bytes, the error is dropped.
 func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
 	if len(buf) < min {
 		return 0, ErrShortBuffer
@@ -323,6 +324,7 @@ func ReadAtLeast(r Reader, buf []byte, min int) (n int, err error) {
 // If an EOF happens after reading some but not all the bytes,
 // ReadFull returns ErrUnexpectedEOF.
 // On return, n == len(buf) if and only if err == nil.
+// If r returns an error having read at least len(buf) bytes, the error is dropped.
 func ReadFull(r Reader, buf []byte) (n int, err error) {
 	return ReadAtLeast(r, buf, len(buf))
 }