bpo-39712: Add doc refs for env. vars & -X options

Mention corresponding environment variables in -X option documentation.
Also reference -X options in environment variable documentation.

Author: Erlend E. Aasland

Doc/using/cmdline.rst

@@ -430,15 +430,17 @@ Miscellaneous options
    Reserved for various implementation-specific options.  CPython currently
    defines the following possible values:
 
-   * ``-X faulthandler`` to enable :mod:`faulthandler`;
+   * ``-X faulthandler`` to enable :mod:`faulthandler`. See also
+     :envvar:`PYTHONFAULTHANDLER`.
    * ``-X showrefcount`` to output the total reference count and number of used
      memory blocks when the program finishes or after each statement in the
      interactive interpreter. This only works on debug builds.
    * ``-X tracemalloc`` to start tracing Python memory allocations using the
      :mod:`tracemalloc` module. By default, only the most recent frame is
      stored in a traceback of a trace. Use ``-X tracemalloc=NFRAME`` to start
-     tracing with a traceback limit of *NFRAME* frames. See the
-     :func:`tracemalloc.start` for more information.
+     tracing with a traceback limit of *NFRAME* frames. See
+     :func:`tracemalloc.start` and :envvar:`PYTHONTRACEMALLOC` for more
+     information.
    * ``-X importtime`` to show how long each import takes. It shows module
      name, cumulative time (including nested imports) and self time (excluding
      nested imports).  Note that its output may be broken in multi-threaded
@@ -450,6 +452,7 @@ Miscellaneous options
    * ``-X utf8`` enables the :ref:`Python UTF-8 Mode <utf8-mode>`.
      ``-X utf8=0`` explicitly disables :ref:`Python UTF-8 Mode <utf8-mode>`
      (even when it would otherwise activate automatically).
+     See also :envvar:`PYTHONUTF8`.
    * ``-X pycache_prefix=PATH`` enables writing ``.pyc`` files to a parallel
      tree rooted at the given directory instead of to the code tree. See also
      :envvar:`PYTHONPYCACHEPREFIX`.
@@ -738,16 +741,17 @@ conflict.
    Python memory allocations using the :mod:`tracemalloc` module. The value of
    the variable is the maximum number of frames stored in a traceback of a
    trace. For example, ``PYTHONTRACEMALLOC=1`` stores only the most recent
-   frame. See the :func:`tracemalloc.start` for more information.
+   frame. See the :func:`tracemalloc.start` for more information.  This is
+   equivalent to setting the :option:`-X` ``tracemalloc`` option.
 
    .. versionadded:: 3.4
 
 
 .. envvar:: PYTHONPROFILEIMPORTTIME
 
    If this environment variable is set to a non-empty string, Python will
-   show how long each import takes.  This is exactly equivalent to setting
-   ``-X importtime`` on the command line.
+   show how long each import takes.  This is equivalent to setting the
+   :option:`-X` ``importtime`` option.
 
    .. versionadded:: 3.7
 
@@ -893,6 +897,7 @@ conflict.
    If this environment variable is set to a non-empty string, enable
    :ref:`Python Development Mode <devmode>`, introducing additional runtime
    checks that are too expensive to be enabled by default.
+   This is equivalent to setting the :option:`-X` ``dev`` option.
 
    .. versionadded:: 3.7