Update the documentation about the sign in QElapsedTimer::(m)secsTo

Update the documentation about the sign in QElapsedTimer::(m)secsTo

Since all implementations calculate other - *this, if other has a higher
value (was started later), then the returned value is positive.

The implementations are:
generic: return other.t1 - t1
win: return ticksToNanoseconds(other.t1 - t1) / 1000000
mac: return absoluteToMSecs(other.t1 - t1);
unix: return (other.t1 - t1) * Q_INT64_C(1000) + fraction...

Task-number: QTBUG-25128
Change-Id: Iff0a3460ae9e9d9bdd82fbaad55657fb60e5235a
Reviewed-by: Giuseppe D'Angelo <dangelog@gmail.com>
Reviewed-by: Casper van Donderen <casper.vandonderen@nokia.com>