docs: timekeeping: Use correct prototype for deprecated functions

Use the correct prototypes for do_gettimeofday(), getnstimeofday() and
getnstimeofday64(). All of these returned void and passed the return
value by reference. This should make the documentation of their
deprecation and replacements easier to search for.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200414221222.23996-1-chris.packham@alliedtelesis.co.nz


Signed-off-by: Jonathan Corbet <corbet@lwn.net>