[html-extra] rename html_classes() to html_attr()

[kbond]

This function can be used for any html attribute (ie data-action, data-controller).

[kbond]

If acceptable, I'll update the docs.

[stof]

it can be used for attributes containing a space-separated list. That's not for any attribute out there. So html_attr is too generic for what it does (thus, using the same name than the html_attr escaping strategy can cause confusion)

[kbond]

That's fair, maybe html_join or html_attr_join? But we'd have to configure the separator which I think would be hard (maybe not worth it).

[fabpot]

If we were to rename to function, html_attr_join sounds good to me.

[fabpot]

@kbond If you're ok with the new name, can you update this PR?

[fabpot]

Can you add a note in the main CHANGELOG file and in doc/deprecated.rst?

[kbond]

I'm going to close this for now. I'm now feeling it's not worth the change.