adjtimex.2: Add fields in struct timex description

This patch updates the man page with the new fields added in
struct timex since last edition of the man page.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
adjtimex.2: wfix: clarify init for 'tai' field

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
adjtimex.2: Some rewordings to Laurent Georget's patch

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>