document the convention used for Euler angles (XY'Z'')

in the Placement dialog and the docstrings of the Rotation python class.
fixes #2007

src/Base/RotationPy.xml

@@ -21,7 +21,7 @@
 				-- a Rotation object
 				-- a Vector (axis) and a float (angle)
 				-- two Vectors (rotation from/to vector)
-				-- three floats (Euler angles) as yaw-pitch-roll
+				-- three floats (Euler angles) as yaw-pitch-roll in XY'Z'' convention
 				-- four floats (Quaternion) where the quaternion is specified as:
 				   q=xi+yj+zk+w, i.e. the last parameter is the real part
 			</UserDocu>
@@ -55,6 +55,7 @@
 				<UserDocu>
 					toEuler(Vector) -> list
 					Get the Euler angles of this rotation
+					as yaw-pitch-roll in XY'Z'' convention
 				</UserDocu>
 			</Documentation>
 		</Methode>