[Rd] Add to Documentation of atan2.

[Stefan Evert]

> On 18 May 2021, at 20:30, Ben Bolker <bbolker using gmail.com> wrote:
> 
>  On my system (Ubuntu), 'man 3 catan' gives documentation on the function, and says "The real part of y is chosen in the interval [-pi/2,pi/2]" - but that _could_ be system-dependent.

My copy of "C in a Nutshell" suggests that this requirement is part of the C99 standard (specified for catanh).

Best,
Stefan