Add some weasel wording about threaded usage of PGresults.

PGresults used to be read-only from the application's viewpoint, but now
that we've exposed various functions that allow modification of a PGresult,
that sweeping statement is no longer accurate.  Noted by Dmitriy Igrishin.

doc/src/sgml/libpq.sgml

@@ -7197,8 +7197,12 @@ int PQisthreadsafe();
   </para>
 
   <para>
-   <structname>PGresult</> objects are read-only after creation, and so
-   can be passed around freely between threads.
+   <structname>PGresult</> 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</>-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</>,
+   too.
   </para>
 
   <para>