pgsql: doc: Reword description of roles able to view track_activities's

doc: Reword description of roles able to view track_activities's info

The information generated when track_activities is accessible to
superusers, roles with the privileges of pg_read_all_stats, as well as
roles one has the privileges of.  The original text did not outline the
last point, while the change done in ac1ae47 was unclear about the
second point.

Per discussion with Nathan Bossart.

Discussion: https://postgr.es/m/20220521185743.GA886636@nathanxps13
Backpatch-through: 10

Branch
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REL_12_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/bc2f04e4f26e303041c72341c0dc2b6be53b156e

Modified Files
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doc/src/sgml/config.sgml | 8 ++++----
1 file changed, 4 insertions(+), 4 deletions(-)