perlxs.pod: update FALLBACK: keyword

Author: iabyn

dist/ExtUtils-ParseXS/lib/perlxs.pod

@@ -1892,20 +1892,22 @@ next keyword or possible start of a new XSUB (C</\n\n\S/>).
 
 =head3 The FALLBACK: Keyword
 
-In addition to the OVERLOAD keyword, if you need to control how
-Perl autogenerates missing overloaded operators, you can set the
-FALLBACK keyword in the module header section, like this:
+    MODULE = Foo PACKAGE = Foo::Bar
 
-    MODULE = RPC  PACKAGE = RPC
+    FALLBACK: TRUE | FALSE | UNDEF
 
-    FALLBACK: TRUE
-    ...
+Since 5.8.1.
+
+It defaults to C<UNDEF> for each package. It sets the default fallback
+handling behaviour for overloaded methods in the current package (i.e.
+C<Foo:Bar> in the example above). It is analogous to the Perl-level:
+
+    package Foo::Bar;
+    use overload "fallback" => 1 | 0 | undef;
 
-where FALLBACK can take any of the three values TRUE, FALSE, or
-UNDEF.  If you do not set any FALLBACK value when using OVERLOAD,
-it defaults to UNDEF.  FALLBACK is not used except when one or
-more functions using OVERLOAD have been defined.  Please see
-L<overload/fallback> for more details.
+It only has any effect if there ends up being at least one XSUB in the
+current package with the L<OVERLOAD|/The OVERLOAD: Keyword> keyword
+present. See L<overload/fallback> for more details.
 
 =head2 The Structure of an XSUB