Clarify usage of encrypt arg (#43330)

According to the do_encrypt interface, encrypt arg should be the hash method name used for encrypting returning password. But in the doc and lookup code it's a boolean flag, correct it to string.

test/units/plugins/lookup/test_password.py

@@ -333,7 +333,7 @@ class TestFormatContent(unittest.TestCase):
         self.assertEqual(
             password._format_content(password=u'hunter42',
                                      salt=None,
-                                     encrypt=False),
+                                     encrypt=None),
             u'hunter42')
 
     def test_encrypt(self):