How to apply a signature as watermark in Lightroom mobile?

The iPhone export process is only slightly different to the Android version which that Adobe document linked above relates to. On the iPhone you have various different export "types", most of which can use different settings, i.e. they will each use their own previous settings. You can change the settings using the settings icon at the right (of Share to...., Open in...., Export to Camera Roll, and Export to Files), and when you tap on the settings icon you will see the "Include Watermark" option. You can also use the "Export as..." option, which doesn't use the last-use settings, instead the individual settings are required each time you use this option. But again, once selected the "Include Watermark" option is available.

Of course, if you are exporting using the "Original" file type, no settings are possible as that option merely copies the original file.