When txn savepoint machinery was implemented, its functions were marked
with API_EXPORT qualifier and they were added to `extra/exports` list.
However, they were decalred out of `cond public` scope, so they weren't
exported actually - the commit fixes that mistake.

Along the way, the commit covers exported functions controlling
transaction flow with tests.

Closes #11731

@TarantoolBot document

Title: Fix txn savepoint methods in C API
Since: 3.2.3, 3.3.4, 3.4.2, 3.5.1, 3.6.0

Current documentation contains incorrect definitions of savepoint
related functions. The correct ones are:

```c
API_EXPORT box_txn_savepoint_t *
box_txn_savepoint(void);

API_EXPORT int
box_txn_rollback_to_savepoint(box_txn_savepoint_t *savepoint);
```

Also, these functions were not exported by mistake. So we should delete
them from docs of old Tarantool versions or write a note explaining that
these methods were forgotten and one can find them in newer versions.

(cherry picked from commit 8a1dad8)