doc: document that property names are unique
Both queries and definitions only support each individual name appearing once.
It is an error to have a name appear more than once.
@@ -41,7 +41,8 @@ property names like
A I<property> is a I<name=value> pair.
A I<property definition> is a sequence of comma separated properties.
-There can be any number of properties in a definition.
+There can be any number of properties in a definition, however each name must
+be unique.
For example: "" defines an empty property definition (i.e., no restriction);
"my.foo=bar" defines a property named I<my.foo> which has a string value I<bar>
and "iteration.count=3" defines a property named I<iteration.count> which
@@ -68,6 +69,7 @@ Matching such clauses is not a requirement, but any additional optional
match counts in favor of the algorithm.
More details about that in the B<Lookups> section.
A I<property query> is a sequence of comma separated property query clauses.
+It is an error if a property name appears in more than one query clause.
The full syntax for property queries appears below, but the available syntactic
features are:
