Missing comments/docs about custom scan path

Hi,

While working on [1], I noticed $SUBJECT: commit e7cb7ee14 failed to
update comments for the CustomPath struct in pathnodes.h, and commit
f49842d1e failed to update docs about custom scan path callbacks in
custom-scan.sgml, IIUC.  Attached are patches for updating these,
which I created separately for ease of review (patch
update-custom-scan-path-comments.patch for the former and patch
update-custom-scan-path-docs.patch for the latter).  In the second
patch I used almost the same text as for the
ReparameterizeForeignPathByChild callback function in fdwhandler.sgml.

Best regards,
Etsuro Fujita

[1] https://www.postgresql.org/message-id/CAPmGK16ah9JtyVPtdqu6d%3DQGkRX%3DRAzoYQfX7%3DLZ%2BKnqwBfftg%40mail.gmail.com