Document `attr` and `attribute`

[hugovk]

Let's update https://devguide.python.org/documentation/markup/ to document that:

• .. attribute:: my-attr should be used for defining attributes, and
• :attr:`my-attr` used for referring to it.

[kokoavailable]

can i work on this issue ? please assign it to me! is there a deadline?

[hugovk]

Yes, you can work on it :) We don't normally assign issues, and don't have a deadline, but someone else can have a go if this is still open after a few weeks.

Let me know if you've any questions.

Thank you!

[kokoavailable]

@hugovk

I've submitted a PR.

#1311

Please let me know If there are any inaccuracies or misunderstanding in my request.

Thank you!