add 'output.errorfile' config option

docs/configuration.rst

@@ -4408,6 +4408,21 @@ Description
     The default format string here is ``"{message}"``.
 
 
+output.errorfile
+----------------
+Type
+    |Path|_
+Description
+    File to write input URLs which returned an error to.
+
+    When combined with
+    ``-I``/``--input-file-comment`` or
+    ``-x``/``--input-file-delete``,
+    this option will cause all input URLs from these files
+    to be commented/deleted after processing them
+    and not just successful ones.
+
+
 output.num-to-str
 -----------------
 Type