bug#9300: 24.0.50; `bounds-of-thing-at-point' does not return nil when just after THING

[Tino Calancha]

> FWIW, I agree with Dmitry: this has been a de-facto behavior long
> enough to consider it the correct one.  If documentation is confusing
> in that it says otherwise, we should fix the documentation.
* If the behaviour is obviously correct the funcs. are easier to use.
* If the definition allows for more general uses, as Drew explained, 
much better.
* For code where the existence of the THING (exactly) at point really
    matters, the current behaviour may cause undesirable effects:

    * Karen Lisp, the mother of 8 yr old Emily, is enjoying a family 
cruise.
    * Karen just downloaded in her favourite iTHING one new APP based on
      'thingatpt:
      - It shows an alert whenever there is _no_ a boat at point.
      - The point equals Emily position.
    * The ship is not moving: they throw the anchor during the Hawaiian
      dance show, which Karen is watching while Emily went to play.

    1) First, Emily went to the prow to receive some fresh air.
       => App: no alert
    2) Later, Emily went to the middle of the ship following one cat.
       => App: no alert
    3) Finally, she went to the stern to see the dolphins.
       => App: no alert
    4) While Emily was feeding the dolphins with her cheeseburger,
       she fell down to the water.
       => App: no alert!!!

    I)  Hopefully Emily knows how to swim.

    II) Emily would be OK if
        (thing-at-point THING)
        return nil whenever there is no THING at point.