Document "method ^foo" syntax

[JJ]

The problem

it's not commented, or used as an example, and it's been implemented since 2015.03:

https://github.com/rakudo/rakudo/blob/master/docs/ChangeLog#L3706

However, it's not been used until recently when @lizmat has added it to many methods.

Suggestions

Comment the syntax, add examples. This is a blocker for #2673 , where one of the new features depends on this syntax.

[coke]

I think the link to the changelog is broken now - might need to link to a specific version so that the "master" link doesn't move the line out from under us.

[JJ]

I don't now why, I can't edit this issue. When I do, the issue pointing to here shows up. Anyway, here's a link to the commit

[JJ]

Also, not clear if there's anything specific about this syntax. Is it affecting the HOW?