Updated C stack external API comments for doxygen documentation.

Updated C stack external API comments for doxygen documentation.

C stack external header files were missing support for doxygen documentation.
Modified comments, spelling, definitions, alignment and terminology.

Change-Id: Ic777555324c45fe5e9fcd421eac20fc418fe10cc
Signed-off-by: Moumita Ray <moumita.ray@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/1431
Reviewed-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-by: Jon A. Cruz <jonc@osg.samsung.com>
Tested-by: Jon A. Cruz <jonc@osg.samsung.com>