docs: Various cleanup of outdated comments and docstrings (#31754)

- Remove pylint warning suppression from SHAKE-128 hexdigest calls; these
  are no longer needed as of astroid 2.12.12. For background, see issue
  <https://github.com/PyCQA/pylint/issues/4039>. Also, correct a comment
  that referred to SHAKE-256 instead of SHAKE-128.
- Replace "released" with "beta" in several places where there was a copy
  & paste error in dark_lang. Add mention of `beta_lang` to a docstring
  where it was omitted.
- Remove comment regarding Mongo startup; the code it referred to has since
  been removed.
- Fix typos.

common/djangoapps/track/middleware.py

@@ -196,7 +196,7 @@ class TrackMiddleware(MiddlewareMixin):
         # This assumes that session_key is high-entropy and unpredictable (which
         # it should be anyway.)
         #
-        # Use SHAKE256 from SHA-3 hash family to generate a hash of arbitrary
+        # Use SHAKE-128 from SHA-3 hash family to generate a hash of arbitrary
         # length.
         hasher = hashlib.shake_128()
         # This is one of several uses of SECRET_KEY.
@@ -209,9 +209,7 @@ class TrackMiddleware(MiddlewareMixin):
         # heads-up to data researchers.
         hasher.update(settings.SECRET_KEY.encode())
         hasher.update(session_key.encode())
-        # pylint doesn't know that SHAKE's hexdigest takes an arg:
-        # https://github.com/PyCQA/pylint/issues/4039
-        return hasher.hexdigest(16)  # pylint: disable=too-many-function-args
+        return hasher.hexdigest(16)
 
     def get_user_primary_key(self, request):
         """Gets the primary key of the logged in Django user"""