[gui] Explain what the "share xpub" menu does

[kloaec]

It's very unclear what the share xPub menu does, especially for a newbie.
Here is a suggestion:

"Use this menu to get your Signing Device xPub. This is useful if someone else is creating a Liana wallet remotely, and want to add your key to their setup.
You don't need to use this menu if you are setting up Liana on this computer."