Add info about RunEnvironmentInfo to binary/env.html

src/main/java/com/google/devtools/build/docgen/templates/attributes/binary/env.html

@@ -8,7 +8,12 @@
 
 <p>
   This attribute only applies to native rules, like <code>cc_binary</code>, <code>py_binary</code>,
-  and <code>sh_binary</code>.  It does not apply to Starlark-defined executable rules.
+  and <code>sh_binary</code>.  It does not apply to Starlark-defined executable rules. For your own
+  Starlark rules, you can add an "env" attribute and use it to populate a
+
+    <a href="/rules/lib/providers/RunEnvironmentInfo.html">RunEnvironmentInfo</a>
+
+  Provider.
 </p>
 
 <p>