go spec: import path implementation restriction

R=rsc, r, r, adg
CC=golang-dev
https://golang.org/cl/5688074

doc/go_spec.html

 <!--{
 	"Title": "The Go Programming Language Specification",
-	"Subtitle": "Version of February 22, 2012"
+	"Subtitle": "Version of February 23, 2012"
 }-->
 
 <!--
@@ -5096,6 +5096,15 @@ it is typically a substring of the full file name of the compiled
 package and may be relative to a repository of installed packages.
 </p>
 
+<p>
+Implementation restriction: A compiler may restrict ImportPaths to
+non-empty strings using only characters belonging to
+<a href="http://www.unicode.org/versions/Unicode6.0.0/">Unicode's</a>
+L, M, N, P, and S general categories (the Graphic characters without
+spaces) and may also exclude the ASCII characters
+<code>!"#$%&amp;'()*,:;&lt;=&gt;?[\]^`{|}</code>.
+</p>
+
 <p>
 Assume we have compiled a package containing the package clause
 <code>package math</code>, which exports function <code>Sin</code>, and