[perl #131239] where clause thunk's return value used for smartmatching

[Zoffix Znet via RT]

On Mon, 01 May 2017 10:15:03 -0700, 1parr...@gmail.com wrote:
> If Zoffix gets this confused, maybe the explanation needs some work
> for mere mortals?

Added a note to docs in https://github.com/perl6/doc/commit/80165120fe