bug#21391: 24.5; `thing-at-point' should return a string

[Eli Zaretskii]

> From: Tino Calancha <tino.calancha@gmail.com>
> Date: Fri, 11 Nov 2016 23:13:55 +0900 (JST)
> cc: Tino Calancha <tino.calancha@gmail.com>, dgutov@yandex.ru, 
>     21391@debbugs.gnu.org
> 
> The current implementation makes the things harder to understand.
> Currently one user need to read the source code to know the actual
> type this function returns.

If that's a problem, let's improve the doc string (and the manual, if
needed) to make this more clear.