Fixed haptic documentation typos (thanks, Elias!).

Partially fixes Bugzilla #2701.
Haptic: Deal with negative periodic magnitudes (thanks, Elias!).

A negative periodic magnitude doesn't exist in Windows' and MacOS' FF APIs

The periodic magnitude parameter of the SDL Haptic API is based on the Linux
 FF API, so it means they are not directly compatible:
    'dwMagnitude' is a 'DWORD', which is unsigned.

Fixes Bugzilla #2701.

--HG--
extra : amend_source : eb0b85870149936fd451ddb0662841112ff93d07