Update OR manual for the sound conditioning feature.

Author: Csantucci

Source/Documentation/Manual/sound.rst

@@ -651,4 +651,51 @@ where the number in parenthesis may be anyone from 0 (nothing heard )
 to 100 (internal track sound reproduced at volume as defined in .sms file).  
 
 If the parameter is not present, the internal track sound is 
-reproduced at the volume as defined in .sms file.
+reproduced at the volume as defined in .sms file.
+
+Conditional sound
+=================
+
+In the real world some sounds are present only at a specific time (season and/or 
+time of day) and/or with a specific weather.
+
+OR provides the parameters to actuate that for any set of sound streams. Consider the 
+sound stream here below::
+
+  			Stream (
+				Priority ( 6 )
+				ORTSSeason ( Spring () winter () )
+				ORTSWeather ( rain () )
+				ORTSTimeOfDay ( 7 12 )
+				ORTSTimeOfDay ( 13 20 )
+				Triggers ( 2
+					Variable_Trigger ( Distance_Dec_Past 400.0
+						StartLoop ( 1
+							File ( "Somma_annunci_loud.wav" -1 )
+							SelectionMethod ( SequentialSelection )
+						)
+					)
+					Variable_Trigger ( Distance_Inc_Past 400.0 ReleaseLoopRelease ())
+				)
+
+				VolumeCurve(
+					DistanceControlled
+					CurvePoints ( 4
+	  					 0.0	1.0
+						230.0	1.0
+	  					260.0	0.0
+						2000.0  0.0
+					)
+					Granularity (0.01)
+				)
+			)
+
+
+As can be seen, there are three keywords, that are ORTSSeason, ORTSWeather and 
+ORTSTimeOfDay. If one or more of the three keywords is not present in the stream, 
+the sound does not depend from that keyword.
+
+In the example shown, the sound is played if all conditions are met, that is season 
+is spring or winter and weather is rain and time of day is within one of the two 
+intervals 7-12 or 13-20. There may be as many TimeOfDay lines as wanted, 
+but the granularity is one hour.