docs: Make 100 column limit for code explicit

      The text within a directive block should align with the

      first character of the directive name.

Keep the line length for documentation less than 80 characters to make

it easier for reviewing in GitHub. Long lines because of URL references

are an allowed exception.

Refer to the Zephyr :ref:`coding_style` for additional requirements.

zephyr-app-commands Directive

*****************************

.. _Linux kernel coding style:

   https://kernel.org/doc/html/latest/process/coding-style.html

In general, follow the `Linux kernel coding style`_, with the

following exceptions:

In general, follow the `Linux kernel coding style`_, with the following

exceptions:

* The line length is 100 columns or fewer. In the documentation, longer lines

  for URL references are an allowed exception.

* Add braces to every ``if``, ``else``, ``do``, ``while``, ``for`` and

  ``switch`` body, even for single-line code blocks. Use the ``--ignore BRACES``

  flag to make *checkpatch* stop complaining.

  comment, ``//``, is not allowed.

* Use ``/**  */`` for doxygen comments that need to appear in the documentation.

When there are differences between the guidelines above and the formatting

generated by code formatting tools, the guidelines above take precedence.

The Linux kernel GPL-licensed tool ``checkpatch`` is used to check

coding style conformity.