Add documentation of TREE_ADDRESSABLE in FIELD_DECL

From-SVN: r34309

gcc/tree.h

@@ -161,8 +161,8 @@ struct tree_common
    addressable_flag:
 
        TREE_ADDRESSABLE in
-   	   VAR_DECL, FUNCTION_DECL, CONSTRUCTOR, LABEL_DECL, ..._TYPE
-	   IDENTIFIER_NODE
+   	   VAR_DECL, FUNCTION_DECL, FIELD_DECL, CONSTRUCTOR, LABEL_DECL,
+	   ..._TYPE, IDENTIFIER_NODE
 
    static_flag:
 
@@ -471,6 +471,9 @@ extern void tree_class_check_failed PARAMS ((const tree, char,
    So it cannot be in a register.
    In a FUNCTION_DECL, nonzero means its address is needed.
    So it must be compiled even if it is an inline function.
+   In a FIELD_DECL node, it means that the programmer is permitted to
+   construct the address of this field.  This is used for aliasing
+   purposes: see record_component_aliases.
    In CONSTRUCTOR nodes, it means object constructed must be in memory.
    In LABEL_DECL nodes, it means a goto for this label has been seen 
    from a place outside all binding contours that restore stack levels.