syntax: document which nil-able fields cannot be nil

[mvdan]

For example, ParamExp has a Name field of type *Lit, but it can never be nil. The reason it's a pointer is for consistency with all the other literal fields.

[Syndace]

There also seem to be fields that can be nil even though they don't look like it. For example, this code checks Stmt.Cmd for nil, which I (with no experience in Go) would've guessed can't be nil. It would be amazing to have all of that clearly documented :)