Add list of attribute target constants

I think the constant names are self-explanatory enough to not require additional information

Closes GH-1168.

Author: Firehed

language/attributes.xml

@@ -333,6 +333,18 @@ class MyAttribute
     </para>
    </example>
 
+   <para>The following targets can be specified:</para>
+   
+   <simplelist>
+    <member><constant>Attribute::TARGET_CLASS</constant></member>
+    <member><constant>Attribute::TARGET_FUNCTION</constant></member>
+    <member><constant>Attribute::TARGET_METHOD</constant></member>
+    <member><constant>Attribute::TARGET_PROPERTY</constant></member>
+    <member><constant>Attribute::TARGET_CLASS_CONSTANT</constant></member>
+    <member><constant>Attribute::TARGET_PARAMETER</constant></member>
+    <member><constant>Attribute::TARGET_ALL</constant></member>
+   </simplelist>
+
    <para>
     By default an attribute can only be used once per declaration. If the attribute should be repeatable on declarations it must
     be specified as part of the bitmask to the <literal>#[Attribute]</literal> declaration.