Added more information about the case of the missing lo_truncate

After some ML message and ticket #104.

doc/src/extensions.rst

@@ -105,6 +105,13 @@ functionalities defined by the |DBAPI|_.
         .. |lo_truncate| replace:: `!lo_truncate()`
         .. _lo_truncate: http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-TRUNCATE
 
+        .. warning::
+
+            If Psycopg is built with |lo_truncate| support (i.e. if the
+            :program:`pg_config` used during setup is version >= 8.3), but at
+            runtime an older libpq is found, Psycopg will fail to import.  See
+            :ref:`the lo_truncate FAQ <faq-lo_truncate>` about the problem.
+
     .. method:: close()
 
         Close the object.

doc/src/faq.rst

@@ -161,6 +161,24 @@ I can't compile `!psycopg2`: the compiler says *error: libpq-fe.h: No such file
     You need to install the development version of the libpq: the package is
     usually called ``libpq-dev``.
 
+.. _faq-lo_truncate:
+.. cssclass:: faq
+
+`!psycopg2` raises `!ImportError` with message *_psycopg.so: undefined symbol: lo_truncate* when imported.
+    This means that Psycopg has been compiled with |lo_truncate|_ support,
+    which means that the libpq used at compile time was version >= 8.3, but at
+    runtime an older libpq library is found. You can use::
+
+        $ ldd /path/to/packages/psycopg2/_psycopg.so | grep libpq
+
+    to find what is the version used at runtime.
+
+    You can avoid the problem by using the same version of the
+    :program:`pg_config` at install time and the libpq at runtime.
+
+    .. |lo_truncate| replace:: `!lo_truncate()`
+    .. _lo_truncate: http://www.postgresql.org/docs/current/static/lo-interfaces.html#LO-TRUNCATE
+
 Psycopg raises *ImportError: cannot import name tz* on import in mod_wsgi / ASP, but it works fine otherwise.
     If `!psycopg2` is installed in an egg_ (e.g. because installed by
     :program:`easy_install`), the user running the program may be unable to