Question (potential issue?) related to datasets caching
It would be nice if the documentation elaborated on all the possible values for `download_mode` and/or a link to `datasets.GenerateMode`. 
This info here:
```
    """`Enum` for how to treat pre-existing downloads and data.
    The default mode is `REUSE_DATASET_IF_EXISTS`, which will reuse both
    raw downloads and the prepared dataset if they exist.
    The generations modes:
    |                                    | Downloads | Dataset |
    | -----------------------------------|-----------|---------|
    | `REUSE_DATASET_IF_EXISTS` (default)| Reuse     | Reuse   |
    | `REUSE_CACHE_IF_EXISTS`            | Reuse     | Fresh   |
    | `FORCE_REDOWNLOAD`                 | Fresh     | Fresh   |
```
I thought I had disabled datasets caching in my code, as follows:
```
from datasets import set_caching_enabled
...
def main():

    # disable caching in datasets
    set_caching_enabled(False)
```
However, in my log files I see messages like the following:

```
04/07/2021 18:34:42 - WARNING - datasets.builder -   Using custom data configuration default-888a87931cbc5877
04/07/2021 18:34:42 - WARNING - datasets.builder -   Reusing dataset csv (xxxx/cache-transformers/datasets/csv/default-888a87931cbc5877/0.0.0/965b6429be0fc05f975b608ce64e1fa941cc8fb4f30629b523d2390f3c0e1a93
```
Can you please let me know what this reusing dataset csv means? I wouldn't expect any reusing with the datasets caching disabled. Thank you!
Question (potential issue?) related to datasets caching 
 I thought I had disabled datasets caching in my code, as follows:
```
from datasets import set_caching_enabled
...
def main():

    # disable caching in datasets
    set_caching_enabled(False)
```
However, in my log files I see messages like the following:

```
04/07/2021 18:34:42 - WARNING - datasets.builder -   Using custom data configuration default-888a87931cbc5877
04/07/2021 18:34:42 - WARNING - datasets.builder -   Reusing dataset csv (xxxx/cache-transformers/datasets/csv/default-888a87931cbc5877/0.0.0/965b6429be0fc05f975b608ce64e1fa941cc8fb4f30629b523d2390f3c0e1a93
```
Can you please let me know what this reusing dataset csv means? I wouldn't expect any reusing with the datasets caching disabled. Thank you! 
 It would be nice if the documentation elaborated on all the possible values for `download_mode` and/or a link to `datasets.GenerateMode`. 
This info here:
```
    """`Enum` for how to treat pre-existing downloads and data.
    The default mode is `REUSE_DATASET_IF_EXISTS`, which will reuse both
    raw downloads and the prepared dataset if they exist.
    The generations modes:
    |                                    | Downloads | Dataset |
    | -----------------------------------|-----------|---------|
    | `REUSE_DATASET_IF_EXISTS` (default)| Reuse     | Reuse   |
    | `REUSE_CACHE_IF_EXISTS`            | Reuse     | Fresh   |
    | `FORCE_REDOWNLOAD`                 | Fresh     | Fresh   |
```