Fix #7780: napoleon: multiple params declaration was wrongly recognized

So far, napoleon converts multiple paramaters declrarations to single
paramaeter fields (`:param:` and `:type:`) unexpectedly.  As a result,
the output seems broken.

This converts it to multiple parameter fields (a pair of field for each
parameter declration) to build parameter successfully.

Note: The conversion rule is available only when napoleon_use_params=True.

tests/test_ext_napoleon_docstring.py

@@ -1337,6 +1337,32 @@ param1 : :class:`MyClass <name.space.MyClass>` instance
         expected = """\
 :param param1:
 :type param1: :class:`MyClass <name.space.MyClass>` instance
+"""
+        self.assertEqual(expected, actual)
+
+    def test_multiple_parameters(self):
+        docstring = """\
+Parameters
+----------
+x1, x2 : array_like
+    Input arrays, description of ``x1``, ``x2``.
+
+"""
+
+        config = Config(napoleon_use_param=False)
+        actual = str(NumpyDocstring(docstring, config))
+        expected = """\
+:Parameters: **x1, x2** (:class:`array_like`) -- Input arrays, description of ``x1``, ``x2``.
+"""
+        self.assertEqual(expected, actual)
+
+        config = Config(napoleon_use_param=True)
+        actual = str(NumpyDocstring(dedent(docstring), config))
+        expected = """\
+:param x1: Input arrays, description of ``x1``, ``x2``.
+:type x1: :class:`array_like`
+:param x2: Input arrays, description of ``x1``, ``x2``.
+:type x2: :class:`array_like`
 """
         self.assertEqual(expected, actual)