pgsql: Fixes in ALTER SUBSCRIPTION DROP PUBLICATION code

Fixes in ALTER SUBSCRIPTION DROP PUBLICATION code

ALTER SUBSCRIPTION DROP PUBLICATION does not actually support
copy_data option, so remove it from tab completion.

Also, reword the error message that is thrown when all the
publications from a subscription are specified to be dropped.

Also, made few doc and cosmetic adjustments.

Author: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Bharath Rupireddy <bharath.rupireddy@enterprisedb.com>
Reviewed-by: Japin Li <japinli@hotmail.com>
Discussion:
https://www.postgresql.org/message-id/flat/CALDaNm21RwsDzs4xj14ApteAF7auyyomHNnp+NEL-sH8m-jMvQ@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/e59d428f34297cd0eb67e3b4e4b8c8bc58504921

Modified Files
--------------
doc/src/sgml/ref/alter_subscription.sgml   | 19 ++++++++++---------
src/backend/commands/subscriptioncmds.c    |  6 +++---
src/bin/psql/tab-complete.c                |  8 ++++++--
src/test/regress/expected/subscription.out |  2 +-
4 files changed, 20 insertions(+), 15 deletions(-)