add a doc for the [ticket:2631] quirk

doc/build/core/schema.rst

@@ -705,6 +705,11 @@ called out with a marker::
         Column('def', String(20), server_onupdate=FetchedValue())
     )
 
+.. versionchanged:: 0.8.0b2,0.7.10
+    The ``for_update`` argument on :class:`.FetchedValue` is set automatically
+    when specified as the ``server_onupdate`` argument.  If using an older version,
+    specify the onupdate above as ``server_onupdate=FetchedValue(for_update=True)``.
+
 These markers do not emit a "default" clause when the table is created,
 however they do set the same internal flags as a static ``server_default``
 clause, providing hints to higher-level tools that a "post-fetch" of these