Rename #generalizedLocalTx to #localTxForReturnType

Since `#generalizedLocalTx` is not released yet, it's not too late to change the name. I think `#localTxForReturnType` is clearer.

### with type parameter

```scala
//val res: Try[Int] = DB.generalizedLocalTx[Try[Int]] { session =>
val res: Try[Int] = DB.localTxForReturnType[Try[Int]] { session =>
```

### without type parameter

```scala
//val res: Try[Int] = DB.generalizedLocalTx { session =>
val res: Try[Int] = DB.localTxForReturnType { session =>
  Try {
  }
}
```