[HACKERS] Document pgstattuple privileges without ambiguity

Hi,

When installing pgstattuple on 10, the documentation about its
privileges was unclear to me. (Does the pg_stat_scan_tables role get
EXECUTE privileges by default or not?).

By making the privilege paragraph less verbose and a duplicate of the
paragraph used for pgfreespacemap and pgbuffercache we remove the
ambiguity and make the documentation more uniform.

The replacement paragrahp is much less verbose and loses some detailed
pointers (to GRANT syntax), but in this instance I feel less is more.

Regards,

Feike