examples: Point users to custom subdomain init examples

examples/userapi/03_subdomains.ipynb

@@ -234,7 +234,7 @@
    "source": [
     "The Devito `SubDomain` API features two steps. The first of these is to create a subclass of `SubDomain`, which encapsulates the template by which `SubDomain`s should be constructed. This is not tied to a specific `Grid`. On a basic level, this specification is created by defining a suitable `define` method for the subclass.\n",
     "\n",
-    "Note if one intends to introduce a custom `__init__` when subclassing `SubDomain`, then it is necessary for this `__init__` to take the `kwarg` `grid=None`, passing it through to `super().__init__(grid=grid)`. Such a custom `__init__` may be useful in applications such as creating many `SubDomain` instances with similar properties from a single template.\n",
+    "Note if one intends to introduce a custom `__init__` when subclassing `SubDomain`, then it is necessary for this `__init__` to take the `kwarg` `grid=None`, passing it through to `super().__init__(grid=grid)`. Such a custom `__init__` may be useful in applications such as creating many `SubDomain` instances with similar properties from a single template. For examples, see `userapi/04_boundary_conditions`, where it is used extensively. Further examples can be found in `tests/test_subdomains`.\n",
     "\n",
     "Here is an example of how to define a subdomain that spans an entire 3D computational grid (this is similar to how 'domain' is defined internally):"
    ]