Log Message:
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.