Docs: book/import#import-func-code lacks examples of importing FunC functions returning tuples

[anton-trunov]

As Tact does not have tuple types (at least yet), like FunC does, importing such functions is challenging for users:

For instance, how does one import a function like the following?

(int, int) derive_pub_key(priv_key: int);

An obvious attempt for the novice Tact programmer fails:

@name(derive_pub_key)
native derivePubKey(privKey: Int): (Int,Int);   // (Int, Int) is invalid Tact

[novusnota]

Tact won't need tuples IMO as it operates on structs mapped to FunC's tuples.

There's info on that in https://docs.tact-lang.org/book/func#tuple-return-type, however I agree that we should also add a smaller example right there + provide a link to "Compatibility with FunC" page, which has to be enhanced per #1145.

P.S.: In a way, this is similar to TypeScript, where one may explicitly use a class over implicitly derived types from {} objects in function returns. Like, SomeClass(arg1, arg2, arg3) over {arg1, arg2, arg3}, where the runtime type of two latter objects will differ, while the runtime type of two former class instances will be the same.

[novusnota]

Described how to work with the similar issue in: #242

[novusnota]

Tied to #1145 and latest notes there