Sets the dock type for the panel, making the panel "dock" in a certain direction, modifying it's position and size.
You can set the inner spacing of a panel's docking using Panel:DockPadding, which will affect docked child panels, and you can set the outer spacing of a panel's docking using Panel:DockMargin, which affects how docked siblings are positioned/sized.
trueas argument if you need to update immediately)
Example docking including DockMargin. Provided by Walrus Viking in this Facepunch post.
Example showing how multiple docked elements behave.