pgsql: doc: Add link targets to CREATE/ALTER FOREIGN TABLE reference pa

doc: Add link targets to CREATE/ALTER FOREIGN TABLE reference pages

This adds IDs to create_foreign_table.sgml's and
alter_foreign_table.sgml's <varlistentry> and <refsect1>, similar to
other reference pages.

Author: jian he <jian.universality@gmail.com>
Reviewed-by: Quan Zongliang <quanzongliang@yeah.net>
Reviewed-by: wenhui qiu <qiuwenhuifx@gmail.com>
Discussion: /messages/by-id/CACJufxE6fW2jFAyTFWEYdUSDP=9P2yYerdksPTgxqDM4DZvvvw@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/dea0812cda1998d6940a4a27369e998046009ae1

Modified Files
--------------
doc/src/sgml/ref/alter_foreign_table.sgml | 78 +++++++++++++++---------------
doc/src/sgml/ref/create_foreign_table.sgml | 48 +++++++++---------
2 files changed, 63 insertions(+), 63 deletions(-)