Plug-ins: docs say when pixelsOmitted is set

An embedded image also comes with pixelsOmitted: true when its data is missing, its width or height is 0, its data cannot be decoded within the decoder's memory limit, or its dimensions differ from pixelSize. The entry tells plug-ins to check the flag before reading pixelsBase64.

docs/plugins.md

@@ -527,7 +527,11 @@ In this example the character and paragraph styles are cut short; a filter gets
   `pixelsOmitted: false`. A CMYK image's pixels are its inks through the working CMYK profile. The pixels of all the
   images in one input take at most 16 MiB in base64. A larger image, one whose pixels don't fit what the images before
   it in paint order left, and a linked image (`linked: true`) have
-  `pixelsOmitted: true` and no pixels; the host checks an image's size before it decodes the image. The host never
+  `pixelsOmitted: true` and no pixels; the host checks an image's size before it decodes the image. An image of any
+  size also has `pixelsOmitted: true` when its data is missing from the document, when its width or height is 0, when
+  its data cannot be decoded (or needs more than 64 MiB of decoder memory, or is a TIFF the host rejects as too
+  large), or when the dimensions in its data differ from `pixelSize`. A plug-in checks `pixelsOmitted` before it
+  reads `pixelsBase64`, which is present only when `pixelsOmitted` is `false`. The host never
   reads a linked image's file for a filter and never gives its path.
 - A symbol instance (`"symbolInstance"`): `symbolId` is the name of the symbol it shows, with `missing: true` when the
   document has no symbol of that name. `overrides` is `{}`.