Accept negative window position

It seems to work on some window managers.

Fixes #1242 <https://github.com/Genymobile/scrcpy/issues/1242>

app/scrcpy.1

@@ -131,13 +131,13 @@ Set a custom window title.
 .BI "\-\-window\-x " value
 Set the initial window horizontal position.
 
-Default is -1 (automatic).\n
+Default is "auto".\n
 
 .TP
 .BI "\-\-window\-y " value
 Set the initial window vertical position.
 
-Default is -1 (automatic).\n
+Default is "auto".\n
 
 .TP
 .BI "\-\-window\-width " value