Document common failure modes

$ p6doc Str.length

  length

Perl 6 does not have a length function. See chars or elems.

$ p6doc List.reverse 

  reverse

    multi sub    reverse(*@list ) returns List:D
    multi method reverse(List:D:) returns List:D

Returns a list with the same elements in reverse order.

Note that reverse always refers to reversing elements 
of a list; to reverse the characters in a string, use flip.