v3.19.0 (review fixes): expose the menu-select Match case choice; doc/comment fixes

Review of 7250dbd:
- the step editor's menu-select form gains a Match case select
  (Case-insensitive (default) / Case-sensitive); the step summary flags a
  case-sensitive step with (case), the saved params carry
  case_sensitive: true only when chosen, and the docs say the editor
  authors the flag.
- docs/api.md: fix the 'with an optionally' grammar.
- the Variant-2 menu test's comment claimed a contains match while it is an
  exact one; a real contains leg is added ('ыбор' -> item 128).
- timeless rationale for the case-insensitive default; the format note now
  dates the change.
sw cache simple-v389 (index.html/help changed).
Tests: 821 frontend + 726 Python.

tests/test_testscript.py

@@ -588,16 +588,26 @@ class TestRunnerActions(RunnerTestCase):
         {'id': 130, 'text': 'Кофе'}]}
 
     def test_menu_text_selection_matches_variant2_cyrillic(self):
+        # a lowercase exact match resolves by default (case-insensitive)
         scc = FakeScc().push('80C2', '', '9000')
         server = FakeServer(scc)
         server.sim_menu = dict(self.LIVE_MENU)
-        # a lowercase contains match resolves by default (case-insensitive)
         run = self.run_script(server, [
             {'type': 'action', 'kind': 'menu-select',
              'params': {'text': 'кофе'}},
         ])
         self.assertEqual(run['status'], 'ok', run['steps'])
         self.assertIn('D30702020181900182', ''.join(scc.sent))
+        # a lowercase contains match resolves too (item 128)
+        scc = FakeScc().push('80C2', '', '9000')
+        server = FakeServer(scc)
+        server.sim_menu = dict(self.LIVE_MENU)
+        run = self.run_script(server, [
+            {'type': 'action', 'kind': 'menu-select',
+             'params': {'text': 'ыбор', 'mode': 'contains'}},
+        ])
+        self.assertEqual(run['status'], 'ok', run['steps'])
+        self.assertIn('D30702020181900180', ''.join(scc.sent))
         # an explicit case-sensitive spec misses and names the loose match
         server = FakeServer(FakeScc())
         server.sim_menu = dict(self.LIVE_MENU)