Extend and document MojDbShardManagerTest

:Release Notes:
Extended test coverage for interfaces used.
Written test description in comments for test.

:Detailed Notes:

:Testing Performed:
build process

:QA Notes:

:Issues Addressed:
[GF-13647] Docs needed for new test: MojDbShardManagerTest

Open-webOS-DCO-1.0-Signed-off-by: Igor Kuksyuk <ikuksyuk@luxoft.com>

Change-Id: If29f1aaefe9875bbc560c1479e1ec7e7f0d78c06
Reviewed-on: https://g2g.palm.com/3113
Reviewed-by: Build Verification
Reviewed-by: Roger Stringer <roger.stringer@lge.com>
Tested-by: Roger Stringer <roger.stringer@lge.com>