STORE: Add documentation on the expectations for returned names

Returned OSSL_STORE_INFO_NAME typed infos are supposed to be a
canonical URI for the corresponding object.  For example, when using
the 'file' scheme loader, the file name is returned, possibly prefixed
with 'file://

'

Reviewed-by: Ben Kaduk <kaduk@mit.edu>
(Merged from https://github.com/openssl/openssl/pull/3856)