bpo-32043: Rephrase -X dev documentation (#4478)

* should not be more verbose if the code is correct
* enabled checks can be "expensive"

Doc/using/cmdline.rst

@@ -411,8 +411,10 @@ Miscellaneous options
      nested imports).  Note that its output may be broken in multi-threaded
      application.  Typical usage is ``python3 -X importtime -c 'import
      asyncio'``.  See also :envvar:`PYTHONPROFILEIMPORTTIME`.
-   * ``-X dev`` enables the "developer mode": enable debug checks at runtime.
-     Developer mode:
+   * ``-X dev``: enable CPython's "developer mode", introducing additional
+     runtime checks which are too expensive to be enabled by default. It should
+     not be more verbose than the default if the code is correct: new warnings
+     are only emitted when an issue is detected. Effect of the developer mode:
 
      * Warning filters: add a filter to display all warnings (``"default"``
        action), except of :exc:`BytesWarning` which still depends on the
@@ -441,8 +443,7 @@ Miscellaneous options
       The ``-X showalloccount`` option.
 
    .. versionadded:: 3.7
-      The ``-X importtime``, ``-X dev`` and :envvar:`PYTHONPROFILEIMPORTTIME`
-      options.
+      The ``-X importtime`` and ``-X dev`` options.
 
 
 Options you shouldn't use