The rule is to install any custom UNIX utilities in /usr/local, but unfortunately due to the way man is set up, the man pages for things installed in /usr/local may not be found.
The problem is that the man configuration file (/usr/share/misc/man.conf) defines a number of MANPATH_MAP directives. Here are some of them:
MANPATH_MAP /usr/bin /usr/share/man
MANPATH_MAP /usr/sbin /usr/share/man
MANPATH_MAP /usr/local/bin /usr/local/share/man
This means, for instance, that if you type man grep, man examines your PATH variable and finds that the grep executable is in /usr/bin. The MANPATH_MAP directive instructs it to look in the /usr/share/man directory to find the man page for grep. This overrides man's default behavior, which would be to look in the "nearby" directory /usr/man, which does not exist.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20060228203534793