Update domdec.rst (#2322)

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Co-authored-by: Thorsten Hater <[email protected]>

doc/concepts/domdec.rst

@@ -35,13 +35,13 @@ We define some terms as used in the context of connectivity
 .. glossary::
 
    cell_group
-      List of same-kinded cells that share some information. Must not be split
+      Collection of cells with the same kind. Will be simulated as a unit. Must not be split
       across domains.
 
    domain
-      Produced by a `load_balancer`, a list of all `cell_groups`
-      located on the same hardware. A `communicator` deals with the full set of
-      cells of one `domain`.
+      Produced by a ``load_balancer``; a list of all ``cell_groups``
+      located on the same hardware. A ``communicator`` deals with the full set of
+      cells of one ``domain``.
 
    domain_decomposition
       List of domains; distributed across MPI processes.