Doc: add XML ID attributes to <varlistentry> tags for create_foreign_table, alter_foreign_table

hi.

with commit like:
https://git.postgresql.org/cgit/postgresql.git/commit/?id=78ee60ed84bb3a1cf0b6bd9a715dcbcf252a90f5

we can make a url link that points directly to a specific subsection
of the document.
for example:
https://www.postgresql.org/docs/devel/sql-createtable.html#SQL-CREATETABLE-PARMS-LIKE-OPT-COMMENTS
will link to exactly the CREATE TABLE LIKE INCLUDING COMMENTS section.

The attached patch adds XML ID attributes to <varlistentry> tags for
create_foreign_table.sgml,
and alter_foreign_table.sgml.
While working on CREATE TABLE LIKE, I happened to find this issue.
Maybe we can have a way to automate all <varlistentry>, but there are
so many <varlistentry>.