Merge branch 'add-reflashing-warning' into 'master'

Add warning, update USB cable type

See merge request Librem5/developer.puri.sm!355

Development_Environment/Phone/Troubleshooting/Reflashing_the_Phone.rst

 Reflashing the Phone
 ====================
 
-This guide describes how to reflash the system image on your Librem 5 phone from a workstation, using a USB C cable.
+This guide describes how to reflash the system image on your Librem 5 phone from a workstation, using a USB-C cable.
 
 .. contents::
    :local:
@@ -104,8 +104,11 @@ If the phone is reported in this way then you can proceed to flash the image.
 Flash the Image
 ---------------
 
-You should now be ready to flash a new image. First, check that everything is
-ready:
+You should now be ready to flash a new image.
+
+**This will erase the contents of the phone's eMMC, including user data.**
+
+First, check that everything is ready:
 
  * Check that the ``librem5-devkit-tools`` directory is the current directory.
  * The current directory should contain a ``files`` directory containing the ``librem5.img`` and ``u-boot-librem5.imx`` files.