Added docs for List.fmt

Author: Jan-Olof Hendig

doc/Type/List.pod6

@@ -1012,6 +1012,20 @@ Throws an exception if an element can not be coerced into Numeric.
     say (1, "0xff").sum;      # 256
     say sum(0b1111, 5);       # 20
 
+=head2 method fmt
+
+Defined as:
+
+    method fmt($format = '%s', $separator = ' ') returns Str:D
+
+Returns a string where each element in the list has been formatted according
+to C<$format> and where each element is separated by C<$separator>.
+
+For more information about formats strings, see L<sprintf|/routine/sprintf>.
+
+    my @a = 8..11;
+    say @a.fmt('%03d', ',');                          # 008,009,010,011
+
 =end pod
 
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