spec: disallow NUL in source files

R=r
CC=golang-dev
https://golang.org/cl/209042

doc/go_spec.html

@@ -89,6 +89,9 @@ will use the term <i>character</i> to refer to a Unicode code point.
 Each code point is distinct; for instance, upper and lower case letters
 are different characters.
 </p>
+<p>
+Source code must not contain the NUL character (U+0000).
+</p>
 
 <h3 id="Characters">Characters</h3>