Consistency between `dt.cbind()` and `Frame.cbind()`

Currently, `dt.cbind()` returns a new frame by cbinding all the input frames. At the same time `Frame.cbind()` method appends frames to the original one.

This behavior could be quite confusing for users, because they may expect `Frame.cbind()` method to return a new frame also, when we return `None`. While changing API may not be a good idea from the compatibility point of view, we could think about at least issuing a warning when `Frame.cbind()` is called on views.

For example, the following code doesn't modify the original `DT` frame, while some users may think it does
```python
>>> from datatable import dt
>>> DT = dt.Frame([1,2,3])
>>> DT[:, :].cbind(DT)
DatatableWarning: Duplicate column name found, and was assigned a unique name: 'C0' -> 'C1'
>>> DT
| C0
| int32
-- + -----
0 | 1
1 | 2
2 | 3
[3 rows x 1 column]
```

At the same time, this code works perfectly fine
```python
>>> DT.cbind(DT)
>>> DT
| C0 C1
| int32 int32
-- + ----- -----
0 | 1 1
1 | 2 2
2 | 3 3
[3 rows x 2 columns]
```

Since `DT[:, :]` and `DT` essentially represent the same thing, it could be a hard time for users understanding the code above.

It seems that even in our [documentation](https://datatable.readthedocs.io/en/latest/api/dt/cbind.html) we kind of confusing in-place and out-of-place functionality when say

>This function is exactly equivalent to:
> dt.Frame().cbind(*frames, force=force)

Because in reality `dt.cbind()` produces a new frame and it is not equivalent to `dt.Frame().cbind()` that cbinds everything to an empty frame and returns `None`.