Give more information on the Glossary add popup

[emmapeel2]

The different glossary options can be confusing for translators. The 'Terminology' option has a small explanation, but the Forbidden and Untranslatable term do not have any, and translators mistake one for another.

I think a small paragraph could help to choose the right options when adding new terms to the Glossary:

Forbidden: Mark this option for translations that should not be used

Untranslatable: Mark this option if the sentence should stay as in the source language, without change.

[nijel]

#15981