Clarify guidelines in Parser.md (#56987)
Clarify guidelines in Parser.md (#56987)


mavasani/roslyn,weltkante/roslyn,dotnet/roslyn,shyamnamboodiripad/roslyn,mavasani/roslyn,shyamnamboodiripad/roslyn,KevinRansom/roslyn,bartdesmet/roslyn,weltkante/roslyn,CyrusNajmabadi/roslyn,weltkante/roslyn,mavasani/roslyn,bartdesmet/roslyn,KevinRansom/roslyn,jasonmalinowski/roslyn,CyrusNajmabadi/roslyn,KevinRansom/roslyn,jasonmalinowski/roslyn,bartdesmet/roslyn,shyamnamboodiripad/roslyn,CyrusNajmabadi/roslyn,dotnet/roslyn,jasonmalinowski/roslyn,dotnet/roslyn