Add rst/sphinx ".. deprecated::" directive to docs in Deprecating Code guidance

Description
Deprecating code, be it functions and args/options, should involve including something similar to the following within their respective function docstrings:

.. deprecated:: 3002,3001.1
    This function is deprecated. Please use ``new_function()`` instead.

This adds .. deprecated:: Sphinx / rst directives, which render into something like this, using salt.netapi.rest_cherrypy docs as an example, for the expire_responses option:

Deprecated since version 2016.11.9,2017.7.3,2018.3.0: The "expire_responses" configuration setting, which corresponds to the timeout_monitor setting in CherryPy, is no longer supported in CherryPy versions >= 12.0.0.

Source code of above example:

https://github.com/saltstack/salt/blob/1a5ce828ad983e15205a80ae361150069afaebac/salt/netapi/rest_cherrypy/app.py#L109-L117

The docs need to be updated here:

• https://docs.saltstack.com/en/latest/topics/development/deprecations.html
  • Source: https://github.com/saltstack/salt/blob/master/doc/topics/development/deprecations.rst

Additional context

Include examples, and provide best practice for function deprecation vs. arg/option deprecations, perhaps?

Start with doc/topics/development/deprecations.rst and compare its current guidance with the linked salt.netapi.rest_cherrypy example in app.py lines 109-117. Update the documentation to explain the .. deprecated:: directive, include examples, and distinguish function deprecation from argument or option deprecation; the documentation should clearly describe the recommended practice.