Improve the definition of poset, fixes #1285
Improve the definition of poset, fixes #1285

Improve the definition of df-poset in its comments
(to clarify it further).

Signed-off-by: David A. Wheeler <9ae72d22d8b894b865a7a496af4fab6320e6abb2@dwheeler.com>

metamath/set.mm,metamath/set.mm,metamath/set.mm