I found the following information by experimenting with Synfig Studio 1.2.2. Corrections are welcome. Please add this information to the tutorial/Help documentation.
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In the following, I discuss how circles and the circle tool work. I assume much of this applies to other shapes as well.
An ordinary Synfig circle (which is the same as its layer, since creating an object always creates one or more layers) is actually a disk rather than a circle (see Definition of “disk”): a uniform-density set of points within a given distance (radius) from a given center.
So a circle is a filled disk with no outline. It is filled using the fill color. The fill color can be manipulated by an unnamed and untitled control pane located below and as a second part of the Toolbox (which is also untitled). The second part of the toolbox shows two small square swatches of color for the outline and fill, and a larger square swatch showing the fill color.
Various kinds of outlined circles can easily be created, but the circle tool must first be modified to do so:
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Click on the circle tool icon or select it using Menu > Toolbox > Circle Tool.
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In the Circle Tool pane, locate a set of six tiny buttons labelled Layer Type.
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Click on these buttons as needed until two buttons are gray (which means enabled): “Create a circle layer” and “Create an outline layer”.
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In the Canvas pane (titled with the current filename), click where you would like the center of the new circle to be and keep the mouse pressed (drag the mouse) to the desired radius. This must be done accurately (I explain why in 6 below).
Note in the untitled Layers pane that two layers have been created for the circle, both with similar names. One of them is a separate Outline layer, which holds the actual circle (a set of uniform-density points at a fixed distance from the center). The other is the plain circle, its filled disk.
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You can change the thickness of the outline by clicking on the Outline Layer in the untitled Layers pane, then changing the Outline Width entry in the untitled Parameters pane. Here and elsewhere, the default (implicit) units are as specified in Menu > Edit > Preferences > System > Unit System (the default default is pixels).
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Note that an Outline object/layer is composed of a curve having 4 spline vertices, so manually changing the size of an existing circle with an outline is difficult, requiring that the circle and the four spline points of the outline be separately adjusted.
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I hope this information helps others as it would have helped me.