Fix docs for $. replacements

Fixes #401

doc/Language/5to6-perlvar.pod6

@@ -416,8 +416,17 @@ Currently no obvious equivalent.
 
 =item $.
 
-The current line number is returned by the C<.ins> method on the appropriate
-filehandle. E. g. C<$*IN.ins>.
+No direct replacement exists. For L<IO::Path> types L<IO::Handle> types,
+use L<kv> method on the L<Seq> returned from L<lines> to return indexes as well
+as lines. The numbering will start from zero, but it'll be there.
+
+For L<IO::CatHandle> types
+(of which L«C<$*ARGFILES>|/language/variables#index-entry-%24%2AARGFILES» is
+one), you can use L«C<on-switch>|/type/IO::CatHandle#method_on-switch» hook
+to reset line number on handle switch, and increment it manually.
+See also L«C<IO::CatHandle::AutoLines>|https://modules.perl6.org/repo/IO::CatHandle::AutoLines»
+and L«C<LN>|https://modules.perl6.org/repo/LN» modules that simplify this
+operation.
 
 =item $INPUT_RECORD_SEPARATOR