Document that -o is always relative to -d option

doc/manual-src/en/aria2c.rst

@@ -232,8 +232,10 @@ HTTP/FTP/SFTP Options
 
 .. option:: -o, --out=<FILE>
 
-  The file name of the downloaded file. When the
-  :option:`--force-sequential <-Z>` option is used, this option is ignored.
+  The file name of the downloaded file.  It is always relative to the
+  directory given in :option:`--dir <-d>` option.  When the
+  :option:`--force-sequential <-Z>` option is used, this option is
+  ignored.
 
   .. note::
 

src/usage_text.h

@@ -38,9 +38,10 @@
 #define TEXT_DIR                                                        \
   _(" -d, --dir=DIR                The directory to store the downloaded file.")
 #define TEXT_OUT                                                        \
-  _(" -o, --out=FILE               The file name of the downloaded file. When \n" \
-    "                              the -Z option is used, this option will be \n" \
-    "                              ignored.")
+  _(" -o, --out=FILE               The file name of the downloaded file. It is\n" \
+    "                              always relative to the directory given in -d\n" \
+    "                              option. When the -Z option is used, this option\n" \
+    "                              will be ignored.")
 #define TEXT_LOG                                                        \
   _(" -l, --log=LOG                The file name of the log file. If '-' is\n" \
     "                              specified, log is written to stdout.")