Add docstrings to submesh creation in Python interface (#3112)

* Add docstrings to submesh creation

* Apply suggestions from code review

Co-authored-by: Garth N. Wells <gnw20@cam.ac.uk>

* Update documentation and properly wrap submesh maps in nanobind

* Apply suggestions from code review

* Fix typo

* Reflow comment.

* Apply suggestions from code review

Co-authored-by: Garth N. Wells <gnw20@cam.ac.uk>

* Update docs and return type

* Add comma to list

---------

Co-authored-by: Garth N. Wells <gnw20@cam.ac.uk>
Co-authored-by: Jack S. Hale <mail@jackhale.co.uk>

python/dolfinx/mesh.py

@@ -40,6 +40,7 @@
     "locate_entities_boundary",
     "refine",
     "create_mesh",
+    "create_submesh",
     "Mesh",
     "MeshTags",
     "meshtags",
@@ -439,7 +440,19 @@ def create_mesh(
     return Mesh(mesh, domain)
 
 
-def create_submesh(msh, dim, entities):
+def create_submesh(
+    msh: Mesh, dim: int, entities: npt.NDArray[np.int32]
+) -> tuple[Mesh, npt.NDArray[np.int32], npt.NDArray[np.int32], npt.NDArray[np.int32]]:
+    """Create a mesh with specified entities from another mesh.
+
+    Args:
+        mesh: Mesh to create the sub-mesh from.
+        dim: Topological dimension of the entities in ``msh`` to include in the sub-mesh.
+        entities: Indices of entities in ``msh`` to include in the sub-mesh.
+    Returns:
+        The (1) sub mesh, (2) entity map, (3) vertex map, and (4) node map (geometry).
+        Each of the maps a local index of the sub mesh to a local index of ``msh``.
+    """
     submsh, entity_map, vertex_map, geom_map = _cpp.mesh.create_submesh(
         msh._cpp_object, dim, entities
     )