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(at)gmail(dot)com>
Reviewed-by: Bharath Rupireddy <bharath(dot)rupireddy(at)enterprisedb(dot)com>
Reviewed-by: Japin Li <japinli(at)hotmail(dot)com>
Discussion: https://www.postgresql.org/message-id/flat/CALDaNm21RwsDzs4xj14ApteAF7auyyomHNnp+NEL-sH8m-jMvQ(at)mail(dot)gmail(dot)com

Branch
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master

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

Modified Files
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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(-)