pgsql: doc: PG 16 relnotes: separate out psql \drg item

doc: PG 16 relnotes: separate out psql \drg item

Reported-by: Pavel Luzanov

Discussion: /messages/by-id/29b97504-80a3-fdcc-538e-cadde3d8ecd5@postgrespro.ru

Backpatch-through: 16 only

Branch
------
REL_16_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/e9f535463a12af2d4e7973dbf1db53c7097df31b

Modified Files
--------------
doc/src/sgml/release-16.sgml | 21 +++++++++++++++++++--
1 file changed, 19 insertions(+), 2 deletions(-)