Correct a typo at admin/config section smb

Originaly I wanted to add a info based on a experience I made, but instead of creating a PR, I commited the change immediate where I have to excuse for. This PR is to show what has been changed and a small correction of a typo in the gui version which should be merged to to have everything clean (just a remove of an unnecessary *). Just to say, that I found out the cause of my mishap...

What´s about:

When you use files_externals with smb, you have to fill in the user you want to grant against. In some cases, you will not get a green light even if everything has been entered correctly. What I found out is, that you may have to add the domain name to the user, especially if the server has not joined a domain. The change is a addition and instead typing "user" I needed to type "domain/user". Having this entered properly, the light turned immediate to green and everything worked.

Because this info was nowhere present, I changed following two files to reflect this:

documentation/admin_manual/configuration/
external_storage_configuration.rst
external_storage_configuration_gui.rst

In the non gui version, I additionally added a second example to reflect this properly.