docs: add note

docs/source/simulator/index.rst

@@ -2,20 +2,20 @@
 Simulator
 *********
 
-A `simulator` essentially represents a mapping from an input space :math:`\mathcal{X}`
-to an output space :math:`\mathcal{Y}`. More specifically, given a simulator
-:math:`\mathbf{y}=\mathbf{f}(\mathbf{x})`, it maps a :math:`p`-dimensional input
-:math:`\mathbf{x} \in \mathcal{X} \subset{\mathbb{R}^p}` to a :math:`k`-dimensional output
-:math:`\mathbf{y} \in \mathcal{Y} \subset{\mathbb{R}^k}`.
+Computer simulations are widely used to study real-world systems in many fields.
+Such a simulation model, so-called `simulator`, essentially defines a mapping from
+an input space :math:`\mathcal{X}` to an output space :math:`\mathcal{Y}`.
+More specifically, given a simulator :math:`\mathbf{y}=\mathbf{f}(\mathbf{x})`,
+it maps a :math:`p`-dimensional input :math:`\mathbf{x} \in \mathcal{X} \subset{\mathbb{R}^p}`
+to a :math:`k`-dimensional output :math:`\mathbf{y} \in \mathcal{Y} \subset{\mathbb{R}^k}`.
 The mapping :math:`\mathbf{f}` can vary from simple linear equations which can be analytically
 solved to complex partial differential equations which requires numerical schemes such as
 finite element methods.
 
-This module hosts functionality for running simulators, as well as some
-simulators used to model landslide run-out processes. Currently implemented
-classes are:
+This module hosts simulators and functionality for running simulators. Currently
+implemented classes are:
 
-* :class:`.RunSimulator`: Serial and parallel execution for simulators defined by the user.
+* :class:`.RunSimulator`: Serial and parallel execution of simulators.
 
 * :class:`.MassPointModel`: Mass point model for landslide run-out simulation.
 
@@ -29,3 +29,10 @@ classes are:
     Run Simulator <run_simulator>
     Mass Point Model <mass_point_model>
     Ravaflow Mixture Model <ravaflow24>
+
+.. note::
+   :class:`.MassPointModel` and :class:`.Ravaflow24Mixture` are only relevant if
+   the user wants to perform run-out simulation. :class:`.MassPointModel` is purely
+   Python-based and can be used right away. :class:`.Ravaflow24Mixture` depends on 
+   `r.avaflow 2.4 <https://www.landslidemodels.org/r.avaflow/>`_, which needs to be
+   installed by the user.
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