Document return type of method round(Real:D: $scale = 1)

[0rir]

This started with PR #4447 . Where I naively suggested

 `round( Real:D: $scale = 1 -->Num)`

An issue seems more appropriate as there are seeming concerns regarding underlying behavior. The lack of specified return types in the documentation is a common problem, that was part of my motivation to submit. I find more confusion in that line, but that may be covered elsewhere in the docs.

With thanks, I am tagging #4447 participants as an invitation (@coke, @raiph, and @Util).