Adjust kernel-doc notation to prevent warnings when using -Wall.

namespace_devs.c:76: warning: No description found for return value of 'nd_is_uuid_unique'
namespace_devs.c:343: warning: No description found for return value of 'shrink_dpa_allocation'
namespace_devs.c:668: warning: No description found for return value of 'grow_dpa_allocation'
namespace_devs.c:958: warning: No description found for return value of 'namespace_update_uuid'
namespace_devs.c:1665: warning: Function parameter or member 'nd_mapping' not described in 'create_namespace_pmem'
namespace_devs.c:1665: warning: Excess function parameter 'nspm' description in 'create_namespace_pmem'
namespace_devs.c:1665: warning: No description found for return value of 'create_namespace_pmem'

[iweiny: s/-errno/ERR_PTR(-errno)/]
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Dan Williams <dan.j.williams@intel.com>
Cc: Vishal Verma <vishal.l.verma@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Ira Weiny <ira.weiny@intel.com>
Cc:  <nvdimm@lists.linux.dev>
Reviewed-by: Ira Weiny <ira.weiny@intel.com>
Link: https://lore.kernel.org/r/20231207210545.24056-3-rdunlap@infradead.orgSigned-off-by: Ira Weiny <ira.weiny@intel.com>