Yes, that's correct. The two methods have very different purposes.
The /// <summary></summary>
comments are used to generate XML documentation for your project at compile-time, which is also parsed by Visual Studio for its IntelliSense database.
The [DescriptionAttribute]
provides description text that is used in the designer, most notably at the bottom of the Properties Window.
Microsoft's own Windows Forms library is littered with both these.
I don't know of any way to merge the two, but consider whether you really want them to be exactly the same. In my own class libraries, I often want to display slightly different information in the designer than I want to include in my technical documentation.
As a simple example, I might want to make clear in the designer that this property is not supported under certain versions of Windows, but relegate this information to the <remarks>
section in my tech docs:
/// <summary>
/// Gets or sets a value indicating whether a shield should be displayed
/// on this control to indicate that process elevation is required.
/// </summary>
/// <remarks>
/// The elevation-required shield is only supported under Windows Vista
/// and later. The value of this property will be ignored under earlier
/// operating systems.
/// </remarks>
[Category("Appearance")]
[Description("Displays a shield to indicate that elevation is required. " +
"(Only applies under Windows Vista and later.)")]
public bool ShowShield { get; set; }