KDL spec has potentially confusing comment about empty multiline strings.

This paragraph:

> The first and last Newline can be the same character (that is, empty multi-line strings are legal).

could be misinterpreted as implying that the following does *not* represent an empty string:

```kdl
"""

"""
```

I think the easiest way to clear this up would be to add an example that states that the two are equivalent.