Add a note that test names must be unique.

Author: RKrahl

doc/src/reference.rst

@@ -13,7 +13,8 @@ Reference
     :param name: the name of the test to be used for referencing by
 	dependent tests.  If not set, it defaults to the node ID
 	defined by pytest, that is the name of the test function,
-	extended by the parameters if applicable.
+	extended by the parameters if applicable.  The name must be
+	unique in the scope, which is currently the test module.
     :type name: :class:`str`
     :param depends: dependencies, a list of names of tests that this
         test depends on.  The test will be skipped unless all of the

doc/src/usage.rst

@@ -46,8 +46,10 @@ default for this name is the node ID defined by pytest, that is the
 name of the test function, extended by the parameters if applicable.
 In some cases, it's not easy to predict the names of the node IDs.
 For this reason, the name of the tests can be overridden by an
-explicit `name` argument to the marker.  The following example works
-exactly as the last one, only the test names are explicitely set:
+explicit `name` argument to the marker.  The names must be unique in
+the scope, which is currently the test module.  The following example
+works exactly as the last one, only the test names are explicitely
+set:
 
 .. literalinclude:: ../examples/named.py