Update docs about async tests (#2830)
Update docs about async tests (#2830)

The section warning about async tests for tests involving a database
was a little confusing to people since it's pretty out dated. This
language was introduce almost [3 years ago](https://github.com/phoenixframework/phoenix/commit/2b456abcea782684861f466d932b451b37279945),
back when async tests touching the database _were_ dangerous. Now that
this is no longer the case thanks to Ecto 2.0, this should be updated
in the guides.
pap/phoenix,Kosmas/phoenix,keathley/phoenix,rafbgarcia/phoenix,phoenixframework/phoenix,mobileoverlord/phoenix,Joe-noh/phoenix,optikfluffel/phoenix,mitchellhenke/phoenix,mschae/phoenix,michalmuskala/phoenix,Kosmas/phoenix,dmarkow/phoenix,rafbgarcia/phoenix,rafbgarcia/phoenix,nshafer/phoenix,dmarkow/phoenix,evax/phoenix,jwarwick/phoenix,eksperimental/phoenix,Joe-noh/phoenix,mitchellhenke/phoenix,0x00evil/phoenix,0x00evil/phoenix,phoenixframework/phoenix,nshafer/phoenix,optikfluffel/phoenix,Gazler/phoenix,0x00evil/phoenix,mobileoverlord/phoenix,michalmuskala/phoenix,pap/phoenix,phoenixframework/phoenix,keathley/phoenix,Gazler/phoenix,nshafer/phoenix,jwarwick/phoenix,optikfluffel/phoenix,evax/phoenix,eksperimental/phoenix,mschae/phoenix