feat: better text for news image & size fields

[wesleyboar]

Overview

Improve text about "Main image" for article, and its size.

Important

More text changes are in #1049.

Related

• RT #37827
• testing includes fix: even better text for news image & size fields #1049

Changes

• changed "translation" for English

Testing

1. Have a blog installed.
2. Visit http://localhost:8000/admin/djangocms_blog/blogconfig/1/change/ "Layouts" section.
   • See image-related text is changed.
   • See image-related text is clearer.
3. Edit any article; open "Image" section.
   • See image-related text is changed.
   • See image-related text is clearer.

UI

	before	after
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