Improve <xref> vs. <command> formatting in the documentation

SQL commands are generally marked up as <command>, except when a link
to a reference page is used using <xref>.  But the latter doesn't
create monospace markup, so this looks strange especially when a
paragraph contains a mix of links and non-links.

We considered putting <command> in the <refentrytitle> on the target
side, but that creates some formatting side effects elsewhere.
Generally, it seems safer to solve this on the link source side.

We can't put the <xref> inside the <command>; the DTD doesn't allow
this.  DocBook 5 would allow the <command> to have the linkend
attribute itself, but we are not there yet.

So to solve this for now, convert the <xref>s to <link> plus
<command>.  This gives the correct look and also gives some more
flexibility what we can put into the link text (e.g., subcommands or
other clauses).  In the future, these could then be converted to
DocBook 5 style.

I haven't converted absolutely all xrefs to SQL command reference
pages, only those where we care about the appearance of the link text
or where it was otherwise appropriate to make the appearance match a
bit better.  Also in some cases, the links where repetitive, so in
those cases the links where just removed and replaced by a plain
<command>.  In cases where we just want the link and don't
specifically care about the generated link text (typically phrased
"for further information see <xref ...>") the xref is kept.

Reported-by: Dagfinn Ilmari Mannsåker <ilmari@ilmari.org>
Discussion: https://www.postgresql.org/message-id/flat/87o8pco34z.fsf@wibble.ilmari.org

doc/src/sgml/ref/create_foreign_table.sgml

@@ -159,7 +159,7 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
       tables from which the new foreign table automatically inherits
       all columns.  Parent tables can be plain tables or foreign tables.
       See the similar form of
-      <xref linkend="sql-createtable"/> for more details.
+      <link linkend="sql-createtable"><command>CREATE TABLE</command></link> for more details.
      </para>
     </listitem>
    </varlistentry>
@@ -171,7 +171,7 @@ CHECK ( <replaceable class="parameter">expression</replaceable> ) [ NO INHERIT ]
       This form can be used to create the foreign table as partition of
       the given parent table with specified partition bound values.
       See the similar form of
-      <xref linkend="sql-createtable"/> for more details.
+      <link linkend="sql-createtable"><command>CREATE TABLE</command></link> for more details.
       Note that it is currently not allowed to create the foreign table as a
       partition of the parent table if there are <literal>UNIQUE</literal>
       indexes on the parent table.  (See also