pgsql: doc: Document commands that cannot be run in a transaction block

doc: Document commands that cannot be run in a transaction block

Mainly covering the new CREATE SUBSCRIPTION and DROP SUBSCRIPTION, but
ALTER DATABASE SET TABLESPACE was also missing.

Branch
------
REL_10_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/e114289e1e4774dfc3371372e36b3f0fed66b741

Modified Files
--------------
doc/src/sgml/ref/alter_database.sgml | 4 ++++
doc/src/sgml/ref/create_subscription.sgml | 10 +++++-----
doc/src/sgml/ref/drop_subscription.sgml | 5 +++++
3 files changed, 14 insertions(+), 5 deletions(-)

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