Guest Author Descriptions template tag

It may be nice to include a template tag to allow display of co-authors with their details, in particular their description (i.e. their bio).

As posted in [this issue on the support forums](https://wordpress.org/support/topic/guest-author-descriptions/), this code should do it:
```php
function coauthors_descriptions( $between = '

', $betweenLast = '

', $before = '

', $after = '

', $echo = true ) {
return coauthors__echo( 'get_the_author_meta', 'tag', array(
'between' => $between,
'betweenLast' => $betweenLast,
'before' => $before,
'after' => $after,
), 'description', $echo );
}
```

Start by locating the existing coauthors__echo helper and related author template tags in the plugin. Use the proposed coauthors_descriptions signature and the support-forum example as the reference, then verify that the tag returns or echoes guest-author descriptions with the requested separators and wrappers.

Written by the indexing model from the issue text.