Document `IMemoryDomains` invariants

src/BizHawk.Emulation.Common/Interfaces/Services/IMemoryDomains.cs

@@ -17,10 +17,16 @@ namespace BizHawk.Emulation.Common
 	{
 		MemoryDomain? this[string name] { get; }
 
+		/// <remarks>implementation note: must also be in the collection</remarks>
 		MemoryDomain MainMemory { get; }
 
 		bool HasSystemBus { get; }
 
+		/// <remarks>
+		/// if <see cref="HasSystemBus"/> is <see langword="false"/>, the getter behaviour is undefined
+		/// (it may return <see cref="MainMemory"/> or another domain)<br/>
+		/// implementation note: must also be in the collection
+		/// </remarks>
 		MemoryDomain SystemBus { get; }
 
 		bool Has(string name);