$ man man
. . .
man uses a sophisticated method of finding manual page files, based on
the invocation options and environment variables, the /pri-
vate/etc/man.conf configuration file, and some built in conventions and
heuristics.
. . .
$ fgrep local/share/man /private/etc/man.conf
MANPATH /usr/local/share/man
MANPATH_MAP /usr/local/bin /usr/local/share/man
MANPATH_MAP /usr/local/sbin /usr/local/share/man
$
(OS X 10.8.n, for some unannounced future value of n, but AFAIK this behavior is unchanged since 10.4 Tiger)
