builtin: document appending a string to a byte slice
Fixes #4873.
R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7421043
diff --git a/src/pkg/builtin/builtin.go b/src/pkg/builtin/builtin.go
index 91d263a..7b5e9ab 100644
--- a/src/pkg/builtin/builtin.go
+++ b/src/pkg/builtin/builtin.go
@@ -114,6 +114,8 @@
// result of append, often in the variable holding the slice itself:
// slice = append(slice, elem1, elem2)
// slice = append(slice, anotherSlice...)
+// As a special case, it is legal to append a string to a byte slice, like this:
+// slice = append([]byte("hello "), "world"...)
func append(slice []Type, elems ...Type) []Type
// The copy built-in function copies elements from a source slice into a
