www_text/mirror: spell out "external index" and "public inbox"
"extindex" and "public-inbox" are project-specific terms which
are probably unsuitable for folks who are seeing this for the
first time.
Use "public inbox" when referring to actual public inboxes,
since "public-inbox" is merely the name for this particular
implementation and others have adopted the same concept (IMHO
the concept is more important than any particular
implementation).