API Claritity: Documented org.opendaylight.yangtools.yang.model.util.repo

Improved documentation of poorly-documented public APIs in
org.opendaylight.yang.model.util.repo which are actually used by
existing implementations.

Added package-info which describes abstract concepts for this package
which are not materialized in interface or class in this package.

Change-Id: I785a48df0af46d363690ed6c733425f8c1790e0d
Signed-off-by: Tony Tkacik <ttkacik@cisco.com>