Doc: Adjust libpq docs about thread safety.

Describe the situation now that --disable-thread-safety is gone.

Author: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA%2BhUKGLtmexrpMtxBRLCVePqV_dtWG-ZsEbyPrYc%2BNBB2TkNsw%40mail.gmail.com

doc/src/sgml/libpq.sgml

@@ -9236,14 +9236,28 @@ void PQinitSSL(int do_ssl);
   </indexterm>
 
   <para>
-   <application>libpq</application> is reentrant and thread-safe by default.
-   You might need to use special compiler command-line
-   options when you compile your application code.  Refer to your
-   system's documentation for information about how to build
-   thread-enabled applications, or look in
-   <filename>src/Makefile.global</filename> for <literal>PTHREAD_CFLAGS</literal>
-   and <literal>PTHREAD_LIBS</literal>.  This function allows the querying of
-   <application>libpq</application>'s thread-safe status:
+   As of version 17, <application>libpq</application> is always reentrant and thread-safe.
+   However, one restriction is that no two threads attempt to manipulate
+   the same <structname>PGconn</structname> object at the same time. In particular,
+   you cannot issue concurrent commands from different threads through
+   the same connection object. (If you need to run concurrent commands,
+   use multiple connections.)
+  </para>
+
+  <para>
+   <structname>PGresult</structname> objects are normally read-only after creation,
+   and so can be passed around freely between threads.  However, if you use
+   any of the <structname>PGresult</structname>-modifying functions described in
+   <xref linkend="libpq-misc"/> or <xref linkend="libpq-events"/>, it's up
+   to you to avoid concurrent operations on the same <structname>PGresult</structname>,
+   too.
+  </para>
+
+  <para>
+   In earlier versions, <application>libpq</application> could be compiled
+   with or without thread support, depending on compiler options. This
+   function allows the querying of <application>libpq</application>'s
+   thread-safe status:
   </para>
 
   <variablelist>
@@ -9261,29 +9275,12 @@ int PQisthreadsafe();
 
      <para>
       Returns 1 if the <application>libpq</application> is thread-safe
-      and 0 if it is not.
+      and 0 if it is not. Always returns 1 on version 17 and above.
      </para>
     </listitem>
    </varlistentry>
   </variablelist>
 
-  <para>
-   One thread restriction is that no two threads attempt to manipulate
-   the same <structname>PGconn</structname> object at the same time. In particular,
-   you cannot issue concurrent commands from different threads through
-   the same connection object. (If you need to run concurrent commands,
-   use multiple connections.)
-  </para>
-
-  <para>
-   <structname>PGresult</structname> objects are normally read-only after creation,
-   and so can be passed around freely between threads.  However, if you use
-   any of the <structname>PGresult</structname>-modifying functions described in
-   <xref linkend="libpq-misc"/> or <xref linkend="libpq-events"/>, it's up
-   to you to avoid concurrent operations on the same <structname>PGresult</structname>,
-   too.
-  </para>
-
   <para>
    The deprecated functions <xref linkend="libpq-PQrequestCancel"/> and
    <xref linkend="libpq-PQoidStatus"/> are not thread-safe and should not be