MASTER_SITE_OVERRIDE in make.conf

[Robert Downes]

My make.conf file contains the line:

    
MASTER_SITE_OVERRIDE=ftp://ftp.uk.freebsd.org/pub/FreeBSD/ports/distfiles

Someone gave me this line months ago when I asked how to instruct make 
to request files from local FTP servers (rather than dumping all 
requests on the master server).

However, I can not find a description for this line in any official 
literature. I have checked man make.conf, and there seems to be no mention.

Where should I be looking for the definitive description of this 
configuration setting, or has it been removed from recent versions of 
FreeBSD?

-- 
Bob