Doc: update documentation for creating custom scan paths.
Commit
f49842d1e added a new callback for custom scan paths, but missed
updating the documentation.
Back-patch to all supported branches.
Discussion: https://postgr.es/m/CAPmGK15ODkN%2B%3DhkBCufj1HBW0x5OTb65Xuy7ryXchMdiCMpx_g%40mail.gmail.com