Documentation for media_storage_providers

This issue has been migrated from [#7140](https://github.com/matrix-org/synapse/issues/7140).

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I want to use the `s3_storage_provider.S3StorageProviderBackend`.

I configured it and when images are send over my synapse server they are also stored in the s3 bucket.
Here is how i configured it:

```
media_storage_providers:
- module: s3_storage_provider.S3StorageProviderBackend
store_local: False
store_remote: True
store_synchronous: True
config:
bucket: synapse1
endpoint_url: $HIDDEN_ENDPOINT
access_key_id: $HIDDEN_KEY
secret_access_key: $HIDDEN_KEY
```

But the images are still also stored locally on disk at `/var/lib/matrix-synapse/media` and if i remove the folder `/var/lib/matrix-synapse/media` the images are not shown anymore although they are still in the s3 bucket. So it seems to me that the `s3_storage_provider.S3StorageProviderBackend` does nothing but also store the data as a backup option?

I also could not find any documentation about `media_storage_providers` in this repo.
Can someone explain what they are and how they are supposed to work please.

My use case is that i want only to use the S3 Bucket and no local storage. Is this possible with `media_storage_providers`?