RELNOTES: ps(1): Document change in behavior for '-a'/'-A'

Document the practical consequence of change "ps(1): Make '-a' and '-A'
always show all processes" (93a94ce731a8) that specifying '-a'/'-A'
leads to printing all processes regardless of the presence of other
process selection options (except for '-x'/'-X', which command
a filter).

While here, mention a possible practical issue with '-J', as reported by
dch@. This option was introduced by commit "Add -J to filter by
matching jail IDs and names." (13767130c7147ae7182a, r265229), which
unfortunately talked about '-J' being a filter while actually
implementing it as a regular selection option which adds to the
processes to display.

Reviewed by: dch
MFC after: 1 day
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50193

(cherry picked from commit 428200d2f0689aa801f5cab15d3066e2c77825f5)

Commit hash edited on MFC.

(cherry picked from commit f18a49a747f7641e93c24d50207d54e7e612f97d)

Approved by: re (cperciva)