blacklist: Avoid duplicate manual pages in METALOG

Previously, blacklist man pages were just a symlink to their blocklist
counterpart, this in turn installed blocklist man pages twice, and
resulted in a duplicate error when running metalog_reader.lua -c.

Take advantage of the duplication to document nuances in blacklist, such
as the fact that it uses the new database and socket name (blocklist).
Also, note that it has been renamed to blocklist. In the future, it
will help to document its deprecation.

Approved by: emaste (mentor)
Fixes: 7238317403b9 ("blocklist: Rename blacklist to blocklist")
MFC after: 2 days

(cherry picked from commit c6240045536548c22ce40d9ef36c1dc52abcfc9c)