Document schema validation

:Release Notes:

:Detailed Notes:
Create comments for Doxygen in header files.
Document validation internals.

:Testing Performed:
Doxygen generation succeeded

:QA Notes:

:Issues Addressed:
[GF-38364] pbnjson: Document schema validation

Open-webOS-DCO-1.0-Signed-off-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>

Change-Id: Id26d0f0b26d34b3cd31b9309ebdc6247b1e5d062
Reviewed-on: https://g2g.palm.com/3438
Reviewed-by: Build Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Reviewed-by: Alekseyev Oleksandr <alekseyev.oleksandr@lge.com>