Update "Login name" to "User name" & improve documentation

[olivierdagenais]

Now that Visual Studio Online (VSO) support was added, the form validation for the User name field was removed and better documentation for what is expected/possible in that field was added.

Manual testing

Superficial

1. Create an empty freestyle project.
2. Select Team Foundation Server as the Source Code Management.
3. Enter inconsequential values for Server URL and Project path.
4. Enter the following values for User name: EXAMPLE-DOMAIN\user, user@domain.example.com and altcreduser. None of them gave a validation message.
5. Click the question mark to the right of the field. It looked like the following:

Real-world

1. Create an empty freestyle project.
2. Select Team Foundation Server as the Source Code Management.
3. Enter real-world values for Server URL and Project path, pointing to a VSO account: https://automated-testing.visualstudio.com/DefaultCollection and $/, respectively.
4. Enter alternate credentials for User name and User password, then queue a build: success!
5. Go back to edit the job and this time, enter the e-mail address associated with the account as the User name and an All Scopes Personal Access Token (PAT) as the User password, then queue a build: success!

Mission accomplished!