What's the proper way to comment a constructor in a generic class?

You need to use curly braces:

/// <summary>
/// Initializes a new instance of the <see cref="Repository{T}"/> class.
/// </summary>

For each typeparam, just add an additional value in the braces, delimited with a comma.

StyleCop has defined how it should look like.

If the class contains generic parameters, these can be annotated within the cref link using either of the following two formats:

/// <summary>
/// Initializes a new instance of the <see cref="Customer`1"/> class.
/// </summary>
public Customer()
{
}

/// <summary>
/// Initializes a new instance of the <see cref="Customer{T}"/> class.
/// </summary>
public Customer()
{
}