[gobject-introspection] docs: added missing docs for annotations attribute

[Philip Chimento <pchimento src gnome org>]

commit e59d55ff1d66cdaa7d5e6af117c6f1a993e0cd3b
Author: David Bellot <david bellot gmail com>
Date:   Thu Feb 28 17:12:20 2019 +1100

    docs: added missing docs for annotations attribute

 docs/gir-1.2.rnc | 3 +++
 1 file changed, 3 insertions(+)
---
diff --git a/docs/gir-1.2.rnc b/docs/gir-1.2.rnc
index 30746c3c..22f5888f 100644
--- a/docs/gir-1.2.rnc
+++ b/docs/gir-1.2.rnc
@@ -54,8 +54,11 @@ grammar {
     }
 
   Annotation =
+       ## element defining an annotation from the source code, usually a user-defined annotation associated 
to a parameter or a return value
     element attribute {
+         ## name of the attribute
       attribute name { xsd:string },
+         ## value of the attribute
       attribute value { xsd:string }
     }