pw: make manual page more friendly for uid/gid search

pw.8 structure is quite different from usual manual page, especially in
describing -o option usage. Specifically, these paragraphs do not
contain "uid/gid" terms, and have "user id"/"group id" instead,
making searching for "override duplicate safety belt" difficult.
Try to simplify such searches.

Also, clarify uid/gid space between 100 and 1000 as "somewhat special",
as it actually is.

Discussed on: russian telegram FreeBSD group
Reviewed by: eugen, novel
MFC after: 1 week