Additional explanation for the "Refine supergroup" feature

Note, that there has been some confusion in regards to the "Refine supergroup" feature. See here:
JabRef/jabref#4664

Another explanatory sentence has been added, that should make it clear, that "Refine supergroup" is an intersection of group assignments, NOT a union.

en/Groups.md

@@ -79,7 +79,7 @@ In the groups view, dynamic groups are shown in *italics* by default. This can b
 
 ### Hierarchical context
 
-By default, a group is **independent** of its position in the groups tree: When selected, only the group's contents are shown. However, especially when using dynamic groups, it is often useful to define a subgroup that **refines its supergroup**, i.e., when selected, entries contained in both groups are displayed. For example, create a supergroup containing entries with the keyword `distribution` and a subgroup containing entries with the keyword `gauss` that refines this supergroup. Selecting the subgroup now displays entries that match both conditions, i.e. are concerned with Gaussian distributions. By adding another refining subgroup for `laplace` to the original supergroup, the grouping can easily be extended. In the groups tree, refining groups have a special icon (this can be turned off in the preferences).
+By default, a group is **independent** of its position in the groups tree: When selected, only the group's contents are shown. However, especially when using dynamic groups, it is often useful to define a subgroup that **refines its supergroup**, i.e., when selected, entries contained in both groups are displayed. For example, create a supergroup containing entries with the keyword `distribution` and a subgroup containing entries with the keyword `gauss` that refines this supergroup. Selecting the subgroup now displays entries that match both conditions, i.e. are concerned with Gaussian distributions. Note that items that only belong to the subgroup `gauss` will not be shown, i.e. for an item to be displayed when selecting `gauss` it is necessary to be assigned to both the subgroup `gauss` and the supergroup `distribution`. By adding another refining subgroup for `laplace` to the original supergroup, the grouping can easily be extended. In the groups tree, refining groups have a special icon (this can be turned off in the preferences).
 
 The logical complement to a refining group is a group that **includes its subgroups**, i.e. when selected, not only the group's own entries, but also its subgroups' entries are shown. In the groups tree, this type of group has a special icon (this can be turned off in the preferences).