Merge pull request #3714 from JuliaReach/schillic/volume

Revise documentation of `volume`

src/API/Unary/volume.jl

@@ -1,14 +1,25 @@
 """
     volume(X::LazySet)
 
-Compute the volume of a set.
+Compute the volume, or Lebesgue measure, of a set.
 
 ### Input
 
 - `X` -- set
 
 ### Output
 
-A real number representing the volume of `X`.
+A real number representing the Lebesgue measure of `X`.
+
+### Notes
+
+The [Lebesgue measure](https://en.wikipedia.org/wiki/Lebesgue_measure) has the
+following common special cases:
+
+- In 1D, it coincides with the *length*.
+- In 2D, it coincides with the *area* (see also [`area`](@ref)).
+- In 3D, it coincides with the *volume*.
+
+In higher dimensions, it is also known as the *hypervolume* or simply *volume*.
 """
 function volume(::LazySet) end