Warn when parallel restoring a custom dump without data offsets

If pg_dump can't seek on its output stream when writing a dump in the
custom archive format (possibly because you piped its stdout to a file)
it can't update that file with data offsets. These files will often
break parallel restoration. Warn when the user is doing pg_restore on
such a file to give them a hint as to why their restore is about to
fail.

The documentation for pg_restore -j is also updated to suggest that you
dump custom archive formats with the -f option.
---
doc/src/sgml/ref/pg_restore.sgml | 9 +++++++++
src/bin/pg_dump/pg_backup_custom.c | 8 ++++++++
2 files changed, 17 insertions(+)