Chore: Trim thumbnail comments and docstrings

The comments and docstrings around PDF thumbnail generation were wordier than the code they describe. This cuts them back to the essential facts.

docs/configuration.md

@@ -1317,17 +1317,15 @@ valid crontab(5) expression describing when to run.
 
 !!! warning
 
-    This option is deprecated and has no effect. It only applied to PDF
-    thumbnail generation via ImageMagick, which no longer happens. It will be
-    removed in a future release and can be removed from your configuration now.
+    Deprecated and has no effect, since PDF thumbnails no longer use
+    ImageMagick. It will be removed in a future release.
 
 #### [`PAPERLESS_CONVERT_TMPDIR=<path>`](#PAPERLESS_CONVERT_TMPDIR) {#PAPERLESS_CONVERT_TMPDIR}
 
 !!! warning
 
-    This option is deprecated and has no effect. It only applied to PDF
-    thumbnail generation via ImageMagick, which no longer happens. It will be
-    removed in a future release and can be removed from your configuration now.
+    Deprecated and has no effect, since PDF thumbnails no longer use
+    ImageMagick. It will be removed in a future release.
 
 #### [`PAPERLESS_APPS=<string>`](#PAPERLESS_APPS) {#PAPERLESS_APPS}
 

docs/setup.md

@@ -417,10 +417,7 @@ to a positive number to enable polling and disable native filesystem notificatio
         `ExecStart=/opt/paperless/.local/bin/celery --app paperless worker --loglevel INFO`
 
 12. Harden ImageMagick by disabling formats that Paperless-ngx does not use.
-    Most distributions disable PDF processing by default, since PDF documents
-    can contain malware. Paperless-ngx no longer passes PDF documents to
-    ImageMagick, so enabling PDF processing is not required and should be left
-    disabled.
+    PDF processing is not needed and should stay disabled.
 
     Configure the active ImageMagick policy file (commonly
     `/etc/ImageMagick-6/policy.xml` or `/etc/ImageMagick-7/policy.xml`) and