ps.1: update regarding -D option and -p x/d interaction

The -p option does not imply -x, it is merely a different mode that ps
uses. Remove that statement from the -p option, effectively rolling back
d6ae056e9dc96c2db45982ac358ba9ed716a9202.

pstef@ introduced the -D option in 5c0a1c15ff8cb66128f4826ace8ba91e0a31486d
which also turns ps into a similar mode. List the -D option along with
the others in the first sentence of the second paragraph of the
DESCRIPTION section for completeness and correctness sake.

Pointed out by: pstef@
Differential Revision: https://reviews.freebsd.org/D42552