Document subroutines &so and &not

[2colours]

According to the current documentation, so exists as a method and as a prefix operator (i.e a subroutine called &prefix:<so>). However, this doesn't seem to explain why 1 ==> so() works, like a subroutine call, and unlike other unary operators followed by a pair of parentheses. Same stands for not.

It turns out that there is actually a &so subroutine here: https://github.com/rakudo/rakudo/blob/2022.07/src/core.c/Mu.pm6#L1389 - in which case it would be good to cover it with the documentation.

The subroutine &so casually appears in Roast as well, the &not subroutine appears extensively in Roast.