My battery health is decreasing very fast. I bought this iphone on 11th of December and sometimes the battery health itself increase nd now it is just 94% 2-3 days back it is 96% nd some days back it is 98-99%

Be sure to read the link which you have already found. Battery Health varies with usage. You need not worry until it reaches 80%, at which time you will need to make an appointment at the nearest Apple Store for battery replacement. Also check these links...

iPhone Battery and Performance - Apple Support

Batteries - Maximizing Performance - Apple

Which version of iOS are you running?

i had this problem a few weeks ago and i contacted apple and they said if it keeps increasing to get it replaced :)