docs: formats.page, added note about backend for formats.

diff --git a/help/C/formats.page b/help/C/formats.page
index 5181d54a55adab1975c4dfed95b97b335f36463d..290576c64a47851e3cacb6ed4c4c52b732529524 100644 (file)
--- a/help/C/formats.page
+++ b/help/C/formats.page
@@ -35,8 +35,16 @@
  <item><p>Scalable Vector Graphics (.svg)</p></item>
  <item><p>Tagged Image File Format (.tiff, .tif)</p></item>
  <item><p>Other Image Files (.gif, .jpeg, .png)</p></item>
+</list>
+<note>
+<p>
+In some Linux distributions, not all formats are supported by default, so you might not be able to view all the formats listed above.  
+</p>
+<p>
+Support for a format is called a <em>backend</em>.  If <app>Evince</app> gives you the error "Unable to Open Document", you may wish to check whether the backend package for the format is installed.
+</p>
+</note>
 
 
-</list>
 
 </page>