Add warning about losing apps & data.

Several questions are posted each week in irc/discord and reddit about
whether format will remove apps and data. Clarify that it will, and 
recommend backing up

Change-Id: I94b8a1ba86bf48781753508118342bad07dd7318

_includes/templates/device_upgrade.md

@@ -86,6 +86,7 @@ The updater app does not support upgrades from one version of LineageOS to anoth
     * {{ device.recovery_boot }}
 {{ wiping_instructions }}
     {% include alerts/warning.html content="Without this step your device will not boot on the new version!" %}
+    {% include alerts/warning.html content="This step ***will*** wipe your personal data and remove any user-installed apps. If these are important to you, then you should back them up ***before*** performing the upgrade." %}
 {%- if device.uses_twrp != true %}
 6. Click `Advanced`, then `Enable ADB`.
 {%- endif %}