encoding/json: document Indent's preservation of trailing spaces

Fixes #13520.

Change-Id: Ia70cc44be3912167b369d7f74d3436331975c300
Reviewed-on: https://go-review.googlesource.com/17561Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/encoding/json/indent.go

@@ -70,8 +70,12 @@ func newline(dst *bytes.Buffer, prefix, indent string, depth int) {
 // indented line beginning with prefix followed by one or more
 // copies of indent according to the indentation nesting.
 // The data appended to dst does not begin with the prefix nor
-// any indentation, and has no trailing newline, to make it
-// easier to embed inside other formatted JSON data.
+// any indentation, to make it easier to embed inside other formatted JSON data.
+// Although leading space characters (space, tab, carriage return, newline)
+// at the beginning of src are dropped, trailing space characters
+// at the end of src are preserved and copied to dst.
+// For example, if src has no trailing spaces, neither will dst;
+// if src ends in a trailing newline, so will dst.
 func Indent(dst *bytes.Buffer, src []byte, prefix, indent string) error {
 	origLen := dst.Len()
 	var scan scanner