misc/yelp3: add a MESSAGE file referencing MANPATH

Add a note informing users they may need to define MANPATH to search
most non-default locations man pages can be found. The distinction with
yelp3 is that it uses a better means of determining search paths, so
this is less likely to be an issue, but I can still duplicate it in
some pkgsrc environments I have. Addresses PR pkg/34881.


(gutteridge)

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$NetBSD: MESSAGE,v 1.1 2019/01/26 21:24:30 gutteridge Exp $

In order to access man pages in most non-default paths, e.g. under
$LOCALBASE/man, you may need to define a MANPATH environment variable
that explicitly includes all relevant search paths.
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