Eliminate implementation details from definition

The signatures in Rakudo do not match those in the documentation:
https://github.com/rakudo/rakudo/blob/e54885e5e6d71bb6ab6d7f4eb86fe21dd9aee3de/src/core/Str.pm6#L1700
and the tests in roast
https://github.com/perl6/roast/blob/2288d4a3c543d72f822164dad529066ed055a0aa/S32-str/split-simple.t
do not test for returning a Seq, so the fact that they actually return
a Seq must be considered an implementation detail with no place in the documentation.

Author: JJ

doc/Type/Str.pod6

@@ -408,22 +408,22 @@ under its working name of L<parse-names>. This denomination will be removed in t
 
 =for code :method
 multi sub    split(  Str:D $delimiter, Str:D $input, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 =for code :method
 multi sub    split(Regex:D $delimiter, Str:D $input, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 =for code :method
 multi sub    split(List:D $delimiters, Str:D $input, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 =for code :method
 multi method split(Str:D:   Str:D $delimiter, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 =for code :method
 multi method split(Str:D: Regex:D $delimiter, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 =for code :method
 multi method split(Str:D: List:D $delimiters, $limit = Inf,
-  :$skip-empty, :$v, :$k, :$kv, :$p --> Seq)
+  :$skip-empty, :$v, :$k, :$kv, :$p)
 
 Splits a string up into pieces based on delimiters found in the string.