Add context to `val` routine entry
Add context to `val` routine entry

While the use of `Nl` and `No` is clear from context in the full Str
document, in the synthetic document for `val`, it could be read as
implying that the actual characters `N` + `l` or `N` + `o` are legal
numeric literals. Explain that these are actually Unicode categories.

perl6/doc,perl6/doc,tbrowder/doc,perl6/doc,perl6/doc,tbrowder/doc,MasterDuke17/doc,MasterDuke17/doc,perl6/doc,MasterDuke17/doc,tbrowder/doc,tbrowder/doc,MasterDuke17/doc