Documentation: timers: hrtimers: Make hybrid union historical

Documentation: timers: hrtimers: Make hybrid union historical

Non-scalar time was removed from the ktime hybrid union in v3.17, and
the union itself followed suit in v4.10.

Make it clear that ktime_t is always a 64bit scalar type, to avoid
confusing the casual reader.

While at it, fix a spelling mistake.

Fixes: 24e4a8c3e8868874 ("ktime: Kill non-scalar ktime_t implementation for 2038")
Fixes: 2456e855354415bf ("ktime: Get rid of the union")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/59250a3d1c2c827b5c1833169a6e652ca6a784e6.1683021785.git.geert+renesas@glider.be
Signed-off-by: Jonathan Corbet <corbet@lwn.net>