docs: re-document TestAPI and TestShared

I considered merging them, now that (without TestAPIProxy) TestAPI is
the only subclass of TestShared.  But reorganizing the alphabetized
helpers in TestShared versus the strictly-sequenced TestAPI methods can
wait.

client/tests/sdk/test_api.py

@@ -17,8 +17,12 @@
 
 class TestAPI(TestShared):
     """
-    Note that TestShared contains most of the test code, which is shared between
-    API and API Proxy tests.
+    Tests here used to call helper methods (without the `test_` prefix) in
+    `TestShared`, because some were implemented differently in `TestAPIProxy`,
+    which was intended to be run on Qubes with `qrexec` available.  This is now
+    the only test module and path, but the separation from `TestShared` is still
+    useful, since these test methods must be run in order while recording VCR.py
+    cassettes via `utils.VCRAPI`.
     """
 
     @VCRAPI.use_cassette
@@ -168,7 +172,6 @@ def test_get_reply_from_source(self):
     def test_get_all_replies(self):
         super().get_all_replies()
 
-    # This test is materially different in the API & API Proxy versions.
     @VCRAPI.use_cassette
     def test_download_reply(self):
         r = self.api.get_all_replies()[0]

client/tests/sdk/test_shared.py

@@ -11,9 +11,9 @@
 
 class TestShared:
     """
-    Base class for test code that is shared by the API and API proxy tests.
-    Tests in this file should not be prefixed with test_; they are called
-    only from subclasses.
+    Base class for tests against the SDK.  Methods in this file should not be
+    prefixed with `test_`; they are run only from subclasses that define their
+    own `test_` methods to be picked up by pytest.
     """
 
     # ---------------- AUTH VALIDATION ----------------