pkill: better document ancestor-hiding

More prominently note that we'll hide ancestors in the first two
paragraphs that provide an overview of pkill/pgrep. It also seems
worth noting in the description of -a that we won't ever unhide the
pgrep/pkill command itself, as mypid is skipped in all process iteration
by way of PSKIP().

Reviewed by: allanjude, des
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D51009