Doc: clarify introductory description of pg_dumpall.

Doc: clarify introductory description of pg_dumpall.

Add a sentence that describes the parts of a cluster's state that are
*not* included in the output.

Also swap two sentences in the introductory paragraph.  Without that,
it is not clear what the "it" at the beginning of the second sentence
is referring to.  Also add a reference to pg_restore, since not all
output formats are restored with pg_dump.

Also clarify the recently-added text about where different output
formats go, and relocate it above the ancillary text about having
to run as superuser.

Reported-by: Dimitre Radoulov <cichomitiko@gmail.com>
Author: Laurenz Albe <laurenz.albe@cybertec.at>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/CAGJBphSX2oMPPu=VM4U8NP4+qffFH_483tFQCJ_s-mOcN3DLDw@mail.gmail.com