copy.sgml: add XML ID attributes to tags.

hi.

similar to what we did in
https://git.postgresql.org/cgit/postgresql.git/commit/?id=78ee60ed84bb3a1cf0b6bd9a715dcbcf252a90f5

I happened to notice COPY.sgml <refsect1> Parameters section is un-linkable.
The Parameters section in copy.sgml has shorter explanations compared to
create_table.sgml, but it contains a long list of options.

So I think making Each parameter linkable would improve readability.