Update docfx user manual about separated metadata file (#591)
Update docfx user manual about separated metadata file (#591)


dotnet/docfx,superyyrrzz/docfx,dotnet/docfx,LordZoltan/docfx,superyyrrzz/docfx,hellosnow/docfx,hellosnow/docfx,928PJY/docfx,LordZoltan/docfx,LordZoltan/docfx,DuncanmaMSFT/docfx,LordZoltan/docfx,dotnet/docfx,hellosnow/docfx,928PJY/docfx,pascalberger/docfx,superyyrrzz/docfx,pascalberger/docfx,pascalberger/docfx,DuncanmaMSFT/docfx,928PJY/docfx