Clarify the new tds adapter (#3769)
Clarify the new tds adapter (#3769)

There are the older thrid-party adapters and the new one built-in to `ecto_sql`. 
Clarify which one is used in phoenix and provide a link for users to explore more.
phoenixframework/phoenix,nshafer/phoenix,optikfluffel/phoenix,phoenixframework/phoenix,0x00evil/phoenix,0x00evil/phoenix,nshafer/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,michalmuskala/phoenix,rafbgarcia/phoenix,optikfluffel/phoenix,eksperimental/phoenix,jwarwick/phoenix,eksperimental/phoenix,jwarwick/phoenix,michalmuskala/phoenix,0x00evil/phoenix,nshafer/phoenix,phoenixframework/phoenix,rafbgarcia/phoenix