Making Document Windows Dockable

According to the Microsoft Documentation for Visual Studio 2005,

"In Tabbed Mode, you can make document windows dockable or undockable
by selecting or clearing Dockable on the Window menu."

See http://msdn2.microsoft.com/en-us/library/xs71dea4.aspx

I right clicked for a context menu, and I found no "Dockable" option.

I clicked the Window menu on the main menu at the top of Visual Studio
and found no such option.

Is this a mistake on the part of microsoft? Where is this Dockable
option for document windows?

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