Replacement for @raoulb pr#914

See #914

ToDo:
- [x] tests
- [x] final rebase
On Fri, Jul 18, 2014 at 05:11:06PM -0700, raoulb wrote:
>    In sympy/functions/elementary/trigonometric.py:
> 
>  >
>  >      Notes
>  >      =====
>  >
>  > -    * cos(x) will evaluate automatically in the case x
>  > -      is a multiple of pi, pi/2, pi/3, pi/4 and pi/6.
>  > +    cos(x) will evaluate automatically in the case x
>  > +    is a multiple of pi, pi/2, pi/3, pi/4 and pi/6.
>  >
> 
>    Should we explicitely state which values are evaluated or just say 'some
>    values'?
>    I think we can expand the list of these values even further.

"some values" sounds better for me.  All we need: to warn user that
sometimes cos(foo).func!=cos.  But it's ridiculos to quote this
ever groving list in the docstring.