Fix #705 (--argfile weirdness)

docs/content/3.manual/manual.yml

@@ -218,7 +218,9 @@ sections:
         value to the jq program as a predefined variable. If you run jq
         with `--argfile foo bar`, then `$foo` is available in the
         program and has the value resulting from parsing the content of
-        the file named `bar`.
+        the file named `bar`.  The file is slurped.  That means the
+        value loaded will be an array of the top-level JSON values found
+        in the file.
 
   - title: Basic filters
     entries: