Add clarification to Downloader documentation

[GetOutOfMyLab]

The documentation for the Downloader states "That will add a new option that shows up to the launcher's Build Distributions screen, "Game-Only Update for Mobile". Check that, uncheck everything else, and run the build process."

I would suggest that the documentation adds verbiage to ensure that the "Build Updates" option is checked. I took the "uncheck everything else" quite literally and was surprised when the folder was empty after building.