Doc: warn that parallel pg_restore may fail if --no-schema was used.

Doc: warn that parallel pg_restore may fail if --no-schema was used.

If the archive file was made with --no-schema or related options
then it likely does not have enough dependency information to
ensure that parallel restore will choose a workable restore order.
Document this.

In passing, do some minor wordsmithing on new nearby text about
restoring from pg_dumpall archives.

Author: vaibhave postgres <postgresvaibhave@gmail.com>
Reviewed-by: David G. Johnston <david.g.johnston@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAM_eQjzTLtt1X9WKvMV6Rew0UvxT3mmhimZa9WT-vqaPjmXk-g@mail.gmail.com