[docs] Documenting that dis is an implementation detail

[Éric Araujo]

Hello documenters

>From a python-dev thread:

Martin v. Löwis:
> The dis module is deliberately (*) not part of the Python language and
> standard library; it's an implementation detail (as is the func_code
> attribute, and the code object). [...]
>
> (*) Unfortunately, the documentation fails to mention that, probably
> because it's too obvious.

Maciej Fijalkowski
> Even more, Jython and IronPython don't have Python bytecode at all and
> they're considered python implementations.

The dis documentation should mention that it’s not authoritative for
Python, just a tool for CPython. I may be able to propose a patch this
week-end.

Do we need to open a bug report to track other mentions of bytecode that
fail to mention it’s an implementation detail?

Regards