vdl: doc on leading '?' with TypeExpr missing #1
Description
vdl-spec.html specifies that a '?' can immediately precede a type, but there is no (apparent)
documentation that indicates what that means or its impact:
https://github.com/vanadium/vanadium.github.io/blob/master/designdocs/vdl-spec.html#L354
The closest thing I found was:
https://github.com/vanadium/go.ref/blob/master/lib/vdl/codegen/vdlgen/type.go#L49
...robert