Update documentation on fixture being invoked more than once in a scope.

changelog/4058.doc.rst

@@ -0,0 +1 @@
+Update fixture documentation to specify that a fixture can be invoked twice in the scope it's defined for.

doc/en/fixture.rst

@@ -259,6 +259,11 @@ instance, you can simply declare it:
 
 Finally, the ``class`` scope will invoke the fixture once per test *class*.
 
+.. note::
+
+    Pytest will only cache one instance of a fixture at a time.
+    This means that when using a parametrized fixture, it may need to be invoked more than once in the scope it's defined for.
+
 
 ``package`` scope (experimental)
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^