Na página man(1)
man:
-M path, --manpath=path
Specify an alternate manpath to use. By default, man uses man‐
path derived code to determine the path to search. This option
overrides the $MANPATH environment variable and causes option -m
to be ignored.
A path specified as a manpath must be the root of a manual page
hierarchy structured into sections as described in the man-db
manual (under "The manual page system"). To view manual pages
outside such hierarchies, see the -l option.
Então, man -M /usr/local/share/man sync
.