cmd/go: clarify documentation for `-overlay` file additions

[inkeliz]

Currently, it's possible to replace files using -overlay (#39958).

---

However, it only have Replace options:

type OverlayJSON struct {
	Replace map[string]string
}

I would like to suggest a new field, may named as Additions:

type OverlayJSON struct {
        Additions map[string]string
	Replace map[string]string
}

---

That is useful when using code-generation/meta-programming. In those cases, would be possible to create new go files, or even creating files such as assembly code (.s) or creating syso files (.syso), without need to write it into the package directory. Currently, those files must be written into the package itself, but the -overlay could be used instead.

[bcmills]

Replace already does allow additions.

(CC @matloob)

[inkeliz]

I know that issues isn't for questions, but in that case I didn't find a way to "add files". I'm not sure if it only works for .go files, but I tried with assembly (.s) and syso files (.syso), it fails or not work. If the key is the "file destination", it will report that doesn't exist the file. If the "file destination" is the map value, it fails to compile (in case of assembly or, in case of syso, it compiles ignoring the syso).

I'll test again later.