Document "--load-mode" for "pio debug" command

Author: ivankravets

core/userguide/cmd_debug.rst

@@ -59,6 +59,11 @@ to a current working directory (``CWD``).
 
 Process project with a custom :ref:`projectconf`.
 
+.. option::
+    --load-mode
+
+Override :ref:`projectconf_debug_load_mode` option.
+
 .. option::
     --interface