Make a quick copy-editing pass over the 9.1 release notes.

Also remove the material about this being an alpha release.

The notes still need a lot of work, but they're more or less presentable
as a beta version now.

doc/src/sgml/config.sgml

@@ -5461,9 +5461,9 @@ dynamic_library_path = 'C:\tools\postgresql;H:\my_project\lib;$libdir'
        <para>
         Applications that wish to use backslash as escape should be
         modified to use escape string syntax (<literal>E'...'</>),
-        because the default behavior of ordinary strings will change
-        in a future release for SQL compatibility.  This variable can
-        be enabled to help detect applications that will break.
+        because the default behavior of ordinary strings is now to treat
+        backslash as an ordinary character, per SQL standard.  This variable
+        can be enabled to help locate code that needs to be changed.
        </para>
       </listitem>
      </varlistentry>