_content/doc/contribute: clarify that all API changes are significant

Text verbatim from 4fe2f9757f

_content/doc/contribute.html

@@ -369,7 +369,10 @@ and the <a href="https://github.com/golang/go/blob/master/src/cmd/vendor/modules
 </p>
 
 <p>
-Significant changes to the language, libraries, or tools must go
+Significant changes to the language, libraries, or tools
+(which includes API changes in the main repo and all golang.org/x repos,
+as well as command-line changes to the <code>go</code> command)
+must go
 through the
 <a href="/s/proposal-process">change proposal process</a>
 before they can be accepted.