doc/filters: add rotate example using degrees

doc/filters.texi

@@ -6741,6 +6741,12 @@ Rotate the input by PI/6 radians counter-clockwise:
 rotate=-PI/6
 @end example
 
+@item
+Rotate the input by 45 degrees clockwise:
+@example
+rotate=45*PI/180
+@end example
+
 @item
 Apply a constant rotation with period T, starting from an angle of PI/3:
 @example