Mention property where it's indexed. Closes #2018

Author: JJ

doc/Language/objects.pod6

@@ -143,7 +143,7 @@ another class.
 =head2 Attributes
 X<|Attribute> X<|Property> X<|Member> X<|Slot>
 
-Attributes are variables that exist per instance of a class. They are where
+Attributes are variables that exist per instance of a class; when instantiated to a value, the association between the variable and its value is called a property. They are where
 the state of an object is stored. In Perl 6, all attributes are I<private>,
 that means they can be accessed directly only by the class instance itself.
 They are typically declared using the C<has> declarator and the C<!> twigil.