doc: Make libpq documentation navigable between functions

Turn most mentions of libpq functions into links.  At id attributes to
most libpq functions, where not existing yet, so that they can be
linked to.  (In a handful of cases there were problems with the PDF
processing toolchain, so those instances were not changed.)

Author: Fabien COELHO <coelho@cri.ensmp.fr>
Reviewed-by: Peter Eisentraut <peter.eisentraut@2ndquadrant.com>
Discussion: https://www.postgresql.org/message-id/flat/alpine.DEB.2.21.1905121032330.27203@lancre

doc/src/sgml/ecpg.sgml

@@ -5915,7 +5915,7 @@ ECPG = ecpg
      <para>
        <function>ECPGtransactionStatus(const char *<replaceable>connection_name</replaceable>)</function>
        returns the current transaction status of the given connection identified by <replaceable>connection_name</replaceable>.
-       See <xref linkend="libpq-status"/> and libpq's <function>PQtransactionStatus()</function> for details about the returned status codes.
+       See <xref linkend="libpq-status"/> and libpq's <xref linkend="libpq-PQtransactionStatus"/> for details about the returned status codes.
      </para>
    </listitem>
 
@@ -8418,7 +8418,7 @@ if (*(int2 *)sqldata->sqlvar[i].sqlind != 0)
      <term><literal>sqlformat</literal></term>
       <listitem>
        <para>
-        Reserved in Informix, value of <function>PQfformat()</function> for the field.
+        Reserved in Informix, value of <xref linkend="libpq-PQfformat"/> for the field.
        </para>
       </listitem>
      </varlistentry>
@@ -8447,7 +8447,7 @@ if (*(int2 *)sqldata->sqlvar[i].sqlind != 0)
      <term><literal>sqlxid</literal></term>
       <listitem>
        <para>
-        Extended type of the field, result of <function>PQftype()</function>.
+        Extended type of the field, result of <xref linkend="libpq-PQftype"/>.
        </para>
       </listitem>
      </varlistentry>

doc/src/sgml/lobj.sgml

@@ -121,7 +121,7 @@
     If an error occurs while executing any one of these functions, the
     function will return an otherwise-impossible value, typically 0 or -1.
     A message describing the error is stored in the connection object and
-    can be retrieved with <function>PQerrorMessage</function>.
+    can be retrieved with <xref linkend="libpq-PQerrorMessage"/>.
    </para>
 
    <para>