pgsql: doc: Mention pg_read_all_stats in description of track_activitie

doc: Mention pg_read_all_stats in description of track_activities

The description of track_activities mentioned that it is visible to
superusers and that the information related to the current session can
be seen, without telling about pg_read_all_stats.  Roles that are
granted the privileges of pg_read_all_stats can also see this
information, so mention it in the docs.

Author: Ian Barwick
Reviewed-by: Nathan Bossart
Discussion: https://postgr.es/m/CAB8KJ=jhPyYFu-A5r-ZGP+Ax715mUKsMxAGcEQ9Cx_mBAmrPow@mail.gmail.com
Backpatch-through: 10

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/ee27871a8caac3a7fc6ebbe524e9a869a630e42b

Modified Files
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doc/src/sgml/config.sgml | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)