clarified light source grammar in documentation (suggested by William Newman)

doc/main.tex

@@ -109,7 +109,6 @@
     captionpos = b,
     upquote = true,
     literate={*}{{\char42}}1
-             {-}{{\char45}}1
              {\ }{{\copyablespace}}1
 }
 

doc/section_emitters.tex

@@ -13,3 +13,28 @@ types is shown below:
     The arrows indicate the directional distribution of light.
 }
 \end{figure}
+\newpage
+Generally, light sources are specified as children of the \code{<scene>} element; for instance,
+the following snippet instantiates a point light emitter that illuminates a sphere.
+\begin{xml}
+<scene version="0.4.0">
+	<emitter type="point">
+		<spectrum name="intensity" value="1"/>
+		<point name="position" x="0" y="0" z="-2"/>
+	</emitter>
+	<shape type="sphere"/>
+</scene>
+\end{xml}
+An exception to this are \emph{area lights}, which turn a geometric object into a light source.
+These are specified as children of the corresponding \code{<shape>} element.
+\begin{xml}
+<scene version="0.4.0">
+	<shape type="sphere">
+		<emitter type="area">
+			<spectrum name="radiance" value="1"/>
+		</emitter>
+	</shape>
+</scene>
+\end{xml}
+Note the parameter names used to specify the light source power, which reflect
+the different associated physical units.