[UR] Follow up on adapter handle spec changes

[callumfare]

A few minor things came up in review of the initial adapter handle commit (#715):

• The documentation for urInit and urTearDown should be updated. We could use this opportunity to decide if urTearDown is still needed and remove it if necessary.
• Depending on the above we could move or rename runtime.yml to adapter.yml
• The new design for urPlatformGet doesn't match urDeviceGet - it takes a list of adapters rather than a single adapter. This minimized the amount of churn at the time, but we should decide if we want this entry point to be consistent or if we should leave it as is.

[callumfare]

Resolved by #793