Docs: consolidate dependency notes in pg_dump and pg_restore

Docs: consolidate dependency notes in pg_dump and pg_restore

The pg_dump documentation had repetitive notes for the --schema,
--table, and --extension switches, noting that dependent database
objects are not automatically included in the dump.  This commit removes
these notes and replaces them with a consolidated paragraph in the
"Notes" section.

pg_restore had a similar note for -t but lacked one for -n; do likewise.

Also, add a note to --extension in pg_dump to note that ancillary files
(such as shared libraries and control files) are not included in the
dump and must be present on the destination system.

Author: Florents Tselai <florents.tselai@gmail.com>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/284C4D55-4F90-4AA0-84C8-1E6A28DDF271@gmail.com