Document unspecced Cool.path

It's unspecced and never will be, but there's some use of it in the
wild, so document it together with message that it's deprecated and users
should use .IO instead (.IO has slightly different behaviour in that
Cool:U returns an IO::Path:U, rather than stringifying and then blowing
up when trying to make an empty IO::Path).

Listed for deprecation in 6.d-prep in
Raku/6.d-prep@87d4fbacde

Author: zoffixznet

doc/Type/Cool.pod6

@@ -666,6 +666,19 @@ The inverse operation is L<chr|#method chr>.
 
 Mnemonic: returns an ordinal number
 
+=head2 method path
+
+Defined as:
+
+    method path()
+
+B<DEPRECATED>. I<Existed only Rakudo implementation and isn't part of any
+language releaseand. Issues deprecation warnings in 6.d language and will be
+removed  entirely when 6.e language is released.>
+
+Stringifies the invocant and converts it to L<IO::Path> object.
+Use the L«C<.IO method>|/routine/IO» instead.
+
 =head2 routine chr
 
 Defined as: