Moves flat to the page of independent routines

Adds another definition, and also eliminates implementation
details. Refs #2823, #1518

doc/Type/independent-routines.pod6

@@ -926,6 +926,19 @@ sprintf Q:b "<b>%s</b>\n",  "Perl 6"; # RESULT: «<b>Perl 6</b>␤␤»
 sprintf     "<b>\%s</b>\n", "Perl 6"; # RESULT: «<b>Perl 6</b>␤␤»
 sprintf     "<b>%s\</b>\n", "Perl 6"; # RESULT: «<b>Perl 6</b>␤␤»
 
+=head2 sub flat
+
+Defined as:
+
+    multi flat(**@list)
+    multi flat(Iterable \a)
+
+Constructs a list which contains any arguments provided, and returns the result
+of calling the C<.flat> method (L<inherited from C<Any>|/type/Any#method_flat>)
+on that list or C<Iterable>:
+
+    say flat 1, (2, (3, 4), $(5, 6)); # OUTPUT: «(1 2 3 4 (5 6))␤»
+
 =end pod
 
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