Minor documentation changes and code improvements as per guidelines

Minor documentation changes and code improvements as per guidelines

Refactored some of the if conditions from If(ptr == NULL) to if (NULL== ptr)
Added some file name @brief for better doxygen comments

Change-Id: I1348e6e53b986fdcca508f7209ba89f807667af6
Signed-off-by: Ashok Channa <ashok.channa@samsung.com>
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/766
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>