add doc for reserved metadata (#514)
add doc for reserved metadata (#514)

* add doc for reserved metadata

* resolve comments

* resolve comments

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