via ThemeElementCategory, while others do not.
This change uses the existing descriptions to display in the description
box and adds description for ones that doesn't have any entries to
ensure the description area is not left empty and that available
descriptions are consistently utilised.

The main change pertaining to this has been merged in
eclipse.platform.ui and eclipse.platform via
https://github.com/eclipse-platform/eclipse.platform.ui/pull/3666 and
https://github.com/eclipse-platform/eclipse.platform/pull/2453
respectively. One of the remaining part pertaining to the above PR's are
addressed via this change.