docs: Add mention about using USB charger when resetting the filesystem.

It seems the Mac will happily wipe out at least some of the data on
a freshly reset filesytem, if the filesystem was reset while plugged
into the Mac.

docs/general.rst

@@ -46,6 +46,9 @@ The modes are:
    filesystem to its factory state, then boots in safe mode.
 
 If your filesystem becomes corrupt, boot into mode 3 to fix it.
+If resetting the filesystem while plugged into your compute doesn't work,
+you can try doing the same procedure while the board is plugged into a USB
+charger, or other USB power supply without data connection.
 
 Errors: flashing LEDs
 ---------------------