Incorrect docs for pre_do_shortcode_tag and do_shortcode_tag filters

[pento]

The $attr parameter in these filters is listed as an array, but it comes from shortcode_parse_atts(), which can return an array or a string.

[dhanendran]

Uploaded the patch for this ticket.

[dd32]

In 39665:

Shortcodes: Clarify the docs for pre_do_shortcode_tag and do_shortcode_tag.

Props dhanendran.
Fixes #39294.

[dd32]

After chatting with @DrewAPicture I'm punting this to 4.8.

If the docs are critical enough, Scott can manually update it, but it's obscure and trivial enough that I don't expect anyone to really run into it.