Doc: Show option for button label for Azure Oauth configuration

What does this MR do?

Update the documentation of the Microsoft Azure OAuth2 OmniAuth Provider to also include a label for the button.

The default label for the login button is "azure_oauth2", and this is pretty much never what the user wants. The code supports a "label" property (see https://gitlab.com/gitlab-org/gitlab/blob/ee5e940b5ae68fc92d41ae28a66328fb4c6130a3/lib/gitlab/auth/o_auth/provider.rb#L75), and this merge request shows an example of how to use it.

There is no test case for this, as far as I can tell, and I guess there is always the risk that I introduced a typo into the example. I don't think I did, though.

Screenshots

Without label:

With label set to "Active AD" (but of course user can say whatever, e.g. "Corporate directory" or similar)

Related issues

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• If applicable, update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Apply the documentation label.