support doc for field

[luoliwoshang]

https://github.com/goplus/llcppg

For project https://github.com/goplus/llcppg, we need to migrate the doc above the fields in the structs.

type User struct {
    // User ID in the system
    ID int64
    // User's full name
    Name string
    // Email address used for login and notifications
    Email string
}