Documenting MojDbPerfIndexTest

:Release Notes:
Add test description

:Detailed Notes:
DB8 Index Performance Test:
overview, layout, indexes, queries, deletion and update.

:Testing Performed:
not required

:QA Notes:

:Issues Addressed:
GF-7785 MojDbPerfIndex isn't documented

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

Change-Id: I5b2d3740cc9f62d61cf948bb415342f07c5f488a
Reviewed-on: https://g2g.palm.com/2871
Reviewed-by: Build Verification
Reviewed-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Tested-by: Igor Kuksyuk <ikuksyuk@luxoft.com>
Reviewed-by: Roger Stringer <roger.stringer@lge.com>