Document how Avo displays HEIC and other non-web image formats (#660)

* Document how Avo displays HEIC and other non-web image formats

* Name the formats Avo converts instead of pointing at web_image_content_types

Author: adrianthedev

docs/4.0/common/file_other_common.md

@@ -1,3 +1,13 @@
+## Displaying images
+
+Avo displays an attachment straight from storage — PNG, JPEG, GIF, WebP, AVIF, and everything else browsers know how to paint.
+
+Four formats they don't get converted first: `image/heic` and `image/heif` (what an iPhone shoots by default), `image/tiff`, and `image/vnd.adobe.photoshop`. Avo displays those through an Active Storage variant, which renders them as PNG. Only Safari decodes HEIC, so the original would be a broken image in Chrome, Edge, and Firefox.
+
+:::warning
+Converting needs an image processor installed — [libvips](https://github.com/libvips/libvips) built with `libheif`, or ImageMagick with the HEIC delegate. Without one the variant can't be processed and the image won't render. See [Active Storage's requirements](https://guides.rubyonrails.org/active_storage_overview.html#requirements).
+:::
+
 ## Authorization
 
 :::info