Update SQL Server Trace Flag.md

[danrcarollo]

I added clarifying message about using Trace Flag 3895. You should NEVER enable this manually as a Startup Trace flag! It puts the Configuration into an inconsistent state if you later try to disable it! If you add the trace flag, the only way to disable TempDB Memory Optimized MetaData is to first remove the trace flag from the startup parameters and then use either sp_configure or ALTER SERVER CONFIGURATION to set the feature OFF