docs: imaplib: use the reST :class: role, escaped

This is a different approach to f385e44, which was reverted for
creating dangling link references.

By prefixing the reStructuredText role target with a ! we disable
conversion to a link, thereby passing continuous integration checks
even though the referenced class is deliberately absent from the
documentation.

Author: foresto

Doc/library/imaplib.rst

@@ -310,7 +310,7 @@ An :class:`IMAP4` instance has the following methods:
 
 .. method:: IMAP4.idle(dur=None)
 
-   Return an ``Idler``: an iterable context manager implementing the ``IDLE``
+   Return an :class:`!Idler`: an iterable context manager implementing the ``IDLE``
    command as defined in :rfc:`2177`.
 
    The context manager sends the ``IDLE`` command when activated by the
@@ -393,7 +393,7 @@ An :class:`IMAP4` instance has the following methods:
 
    .. note::
 
-      Note: The ``Idler`` class name and structure are internal interfaces,
+      Note: The :class:`!Idler` class name and structure are internal interfaces,
       subject to change.  Calling code can rely on its context management,
       iteration, and public method to remain stable, but should not
       subclass, instantiate, or otherwise directly reference the class.