Add notes about type-family overlap in GHCi (see Trac #7102)

docs/users_guide/ghci.xml

@@ -614,7 +614,7 @@ Prelude>
     <sect2 id="ghci-decls">
       <title>Type, class and other declarations</title>
 
-      <para>[<emphasis role="bold">New in version 7.4.1</emphasis>] At the GHCi
+      <para>At the GHCi
       prompt you can also enter any top-level Haskell declaration,
       including <literal>data</literal>, <literal>type</literal>, <literal>newtype</literal>, <literal>class</literal>, <literal>instance</literal>, <literal>deriving</literal>,
       and <literal>foreign</literal> declarations.  For
@@ -662,6 +662,10 @@ Prelude>
       an attempt to distinguish it from the new <literal>T</literal>,
       which is displayed as simply <literal>T</literal>.</para>
 
+    <para>Class and type-family instance declarations are simply added to the list of available isntances, with one
+    exception. Since type-family instances are not permitted to overlap, but you might want to re-define one,
+    a type-family instance <emphasis>replaces</emphasis> any earlier type instance with an identical left hand side.
+    (See <xref linkend="type-families"/>.)</para>
     </sect2>
 
     <sect2 id="ghci-scope">

docs/users_guide/glasgow_exts.xml

@@ -4891,6 +4891,7 @@ type instance G (a, Int)  = [a]
 type instance G (Char, a) = [a]  -- ILLEGAL overlap, as [Char] /= [Int]
 </programlisting>
       </para>
+    <para> However see <xref linkend="ghci-decls"/> for the overlap rules in GHCi.</para>
     </sect3>
 
     <sect3 id="type-family-decidability">