ClientStorage: Fix thinko in documentation for credentials/username/password/realm

d4805a0f (*: Documentation, Cosmetics) documented all credentials and
username/password/realm as "credentials" and that it is ZEO4-only. It is
not correct however:

- username/password/realm are indeed related to basic authentication
  which, in ZEO5, has been _already_ superseded by SSL. Already because
  those parameters, besides the following assert

	assert not username or password or realm

  are otherwise ignored by ClientStorage constructor.

- credentials however is ZEO5-only thing added in 2016 by Jim in commit
  dbb066d2 (Added the ability to pass credentials when creating
  client storages) with the following commit message:

    Added the ability to pass credentials when creating client storages.

    This is experimental in that passing credentials will cause
    connections to an ordinary ZEO server to fail, but it facilitates
    experimentation with custom ZEO servers. Doing this with custom ZEO
    clients would have been awkward due to the many levels of
    composition involved.

    In the future, we expect to support server security plugins that
    consume credentials for authentication (typically over SSL).

    Note that credentials are opaque to ZEO. They can be any object with
    a true value.  The client mearly passes them to the server, which
    will someday pass them to a plugin.

  To my knowledge there is no use of such "credentials" feature, and
  regular ZEO server will just fail in register if any credentials
  object is passed. Still this feature is separate from ZEO4 basic
  authentication support.

-> Correct the documentation.

src/ZEO/ClientStorage.py

@@ -219,12 +219,15 @@ class ClientStorage(ZODB.ConflictResolution.ConflictResolvingStorage):
             Wait for server connection, defaulting to true.
 
         credentials
+            [Experimental] Credentials object for additional authentication to
+            server.
         username
         password
         realm
-            [ZEO4 only] Credentials for authentication to server.
-            In ZEO5 support for credentials has been dropped in favor of SSL.
-            `credentials` support is scheduled to be removed in `ZEO6`.
+            [ZEO4 only] Credentials for basic authentication to server.
+            In ZEO5 support for basic authentication has been dropped in favor
+            of SSL. Basic authentication support is scheduled to be removed in
+            `ZEO6`.
 
         server_sync
             Whether sync() should make a server round trip, thus causing client