language spec: make NUL byte rule an implementation restriction

R=rsc, r, iant, ken2
CC=golang-dev
https://golang.org/cl/211041

doc/go_spec.html

@@ -90,7 +90,8 @@ 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).
+Implementation restriction: For compatibility with other tools, a
+compiler may disallow the NUL character (U+0000) in the source text.
 </p>
 
 <h3 id="Characters">Characters</h3>