glsl: Add a comment to note what an exec_list is a list of.

Reviewed-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Ian Romanick <ian.d.romainck@intel.com>

src/glsl/ast.h

@@ -526,6 +526,7 @@ public:
 			  struct _mesa_glsl_parse_state *state);
 
    ast_fully_specified_type *type;
+   /** List of 'ast_declaration *' */
    exec_list declarations;
 
    /**