Public/Private folder configuration non-obvious

Moving this to 8.x-3.x as this field does not exist in 4.x.

A complex solution would be to use the fallback to the root folder as described above, although I'm sure there's a good reason for defaulting to 's3fs-public' I'm unaware of.

2 reasons I can think of why we wouldn't want to do a fallback to the root folder are:

• Avoiding cross scheme file access, originally this would have been just to avoid having public://test.txt overwrite s3://test.txt but after SA-CONTRIB-2022-057 it has taken on a more significant role.
• Providing a method for s3fs to know what paths belong to each scheme. In 3.x the s3fs_file table uses the URI not a bucket path to store file metadata, during a cache refresh we need to be able to map the correct scheme based only on the location in the bucket.

Adding the text appears to be a reasonable change to make it self documenting.

Opened MR. Instead of changing the description text I propose that we just populate the field with the default value. This can avoid a translation nullification.

Automatically closed - issue fixed for 2 weeks with no activity.