Added note in documentation for max payload length.

Added note in documentation for max payload length.

This addresses IOT-427.
We were not accounting for json suffixes and prefixes when constructing
in the stack. The C sample apps construct json repsonses too.
After discussions, changed the constants in ocstackconfig.h and
added a note in the documentation to inform api users of the affixes.

Change-Id: I790a6d837a3dabc0d1984cfeb5fb49ef881031e4
Signed-off-by: Mandeep Shetty <mandeep.shetty@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/743
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Doug Hudson <douglas.hudson@intel.com>
Reviewed-by: Erich Keane <erich.keane@intel.com>