Add documentation for GreenzoneInvalidated and update Lua's documentation.

src/BizHawk.Client.Common/movie/interfaces/ITasMovie.cs

@@ -21,6 +21,11 @@ namespace BizHawk.Client.Common
 		int LastEditedFrame { get; }
 		bool LastEditWasRecording { get; }
 
+		/// <summary>
+		/// Called whenever the movie is modified in a way that could invalidate savestates in the movie's state history.
+		/// Called regardless of whether any states were actually invalidated.
+		/// The parameter is the last frame number who's savestate (if it exists) is still valid. That is, the first of the modified frames.
+		/// </summary>
 		Action<int> GreenzoneInvalidated { get; set; }
 
 		string DisplayValue(int frame, string buttonName);

src/BizHawk.Client.EmuHawk/tools/Lua/Libraries/TAStudioLuaLibrary.cs

@@ -544,7 +544,7 @@ namespace BizHawk.Client.EmuHawk
 		}
 
 		[LuaMethodExample("tastudio.ongreenzoneinvalidated( function( currentindex )\r\n\tconsole.log( \"Called whenever the greenzone is invalidated.\" );\r\nend );")]
-		[LuaMethod("ongreenzoneinvalidated", "Called whenever the greenzone is invalidated. Your callback can have 1 parameter, which will be the index of the last row before the invalidated ones.")]
+		[LuaMethod("ongreenzoneinvalidated", "Called whenever the movie is modified in a way that could invalidate savestates in the movie's state history. Called regardless of whether any states were actually invalidated. Your callback can have 1 parameter, which will be the last frame before the invalidated ones. That is, the first of the modified frames.")]
 		public void OnGreenzoneInvalidated(LuaFunction luaf)
 		{
 			if (Engaged())