Give translators more context about how the term 'density' is used

[Stormheg]

When translating I encountered a term that I found very ambiguous.

Normally, more context can be given with a special comment that starts with the Translators keyword. The comment is then pulled into the corresponding django.po file entry. See the Django docs: https://docs.djangoproject.com/en/5.0/topics/i18n/translation/#comments-for-translators

Not sure if Transifex understands comments like this. I believe this is the first time we use this feature in Wagtail.

[squash-labs]

Manage this branch in Squash

Test this branch here: https://stormhegchoretranslation-comme-xpysu.squash.io

[Stormheg]

Fantastic - it shows up on Transifex!