Update documentation on Integral operator to give additional details on different coordinate systems (pybamm-team#4595)

Author: isaacbasil

src/pybamm/expression_tree/unary_operators.py

@@ -611,10 +611,13 @@ class Integral(SpatialOperator):
     A node in the expression tree representing an integral operator.
 
     .. math::
-        I = \\int_{a}^{b}\\!f(u)\\,du,
+        I = \\int_{u_{min}}^{u_{max}}\\!f(u)\\,dq,
 
-    where :math:`a` and :math:`b` are the left-hand and right-hand boundaries of
-    the domain respectively, and :math:`u\\in\\text{domain}`.
+    where :math:`u\\in\\text{domain}` is a spatial variable, :math:`u_{min}` and :math:`u_{max}` are the values of
+    :math:`u` at the left-hand and right-hand boundaries of the domain respectively, and :math:`dq` is given by, \n
+    :math:`dq=du` for cartesian coordinates, \n
+    :math:`dq=2\pi udu` for cylindrical coordinates, \n
+    :math:`dq=4\pi u^2 du` for spherical coordinates.
 
     Parameters
     ----------