crypto/cipher: update docs for the Stream interface

Specify what will happen if len(dst) != len(src).

Change-Id: I66afa3730f637753b825189687418f14ddec3629
Reviewed-on: https://go-review.googlesource.com/1754Reviewed-by: Adam Langley <agl@golang.org>

src/crypto/cipher/cipher.go

@@ -29,6 +29,9 @@ type Block interface {
 type Stream interface {
 	// XORKeyStream XORs each byte in the given slice with a byte from the
 	// cipher's key stream. Dst and src may point to the same memory.
+	// If len(dst) < len(src), XORKeyStream should panic. It is acceptable
+	// to pass a dst bigger than src, and in that case, XORKeyStream will
+	// only update dst[:len(src)] and will not touch the rest of dst.
 	XORKeyStream(dst, src []byte)
 }