mandoc.
If MANPATH begins with a colon, it is appended to the default list; if it ends
with a colon, it is prepended to the default list; or if it contains two
adjacent colons, the standard search path is inserted between the colons. If
none of these conditions are met, it overrides the standard search path.
Import the MANPATH description from mandoc into the man(1) man page
Reported by: kargl
MFC after: 1 week
8013a92f9a