Add @param documentation for ppower in Cylinder.H

Co-authored-by: The9Cat <25960766+The9Cat@users.noreply.github.com>

Author: Copilot

src/Cylinder.H

@@ -117,6 +117,10 @@ class MixtureBasis;
     @param mtype is the deprojected model type for conditioning the
     basis. See ExpCylSL::mtype for options.
 
+    @param ppower is the power-law index for the Power spherical model
+    used as the deprojected model for basis conditioning (default: 2.0).
+    Only used when mtype is set to the Power model.
+
     @param self_consistent set to false turns off potential expansion;
     only performed the first time