pgsql: Doc: Fix various inconsistencies

Doc: Fix various inconsistencies

This fixes multiple areas of the documentation:
- COPY for its past compatibility section.
- SET ROLE mentioning INHERITS instead of INHERIT
- PREPARE referring to stmt_name, that is not present.
- Extension documentation about format name with upgrade scripts.

Backpatch down to 9.4 for the relevant parts.

Author: Alexander Lakhin
Discussion: /messages/by-id/bf95233a-9943-b341-e2ff-a860c28af481@gmail.com
Backpatch-through: 9.4

Branch
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REL9_5_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/6ddc406df055969ee2416449f7e4abdddde53d84

Modified Files
--------------
doc/src/sgml/extend.sgml | 2 +-
doc/src/sgml/ref/copy.sgml | 20 ++++++++++----------
doc/src/sgml/ref/set_role.sgml | 2 +-
3 files changed, 12 insertions(+), 12 deletions(-)