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.
This aligns with the standard usage elsewhere in Eclipse preference
pages.

The main change pertaining to this has been merged in
eclipse.platform.ui via
https://github.com/eclipse-platform/eclipse.platform.ui/pull/3666
and the remaining part is addressed via this change.