get_boundary_post Does not return single post object

[pathawks]

get_boundary_post Seems not to return a single post object, the way get_adjacent_post does.

It seems that get_boundary_post should call array_shift or get_post on the results, in a similar way to get_adjacent_post.

It's just odd that the behavior is different.

[ericlewis]

Although it might be strange, changing the return value here would break any code built with get_boundary_post(). The cost of backwards compatibility is dealing with these odd nuances.

[ericlewis]

We should update the inline documentation here, its incorrect as it stands. See attachment:22950.diff​.

[DrewAPicture]

Docs update looks good.

[DrewAPicture]

In 27235:

Correct PHPDoc return description for get_boundary_post().

Props ericlewis.
Fixes #22950.