update docstring of Size (#1011)

Author: hyrodium

src/traits.jl

@@ -25,8 +25,9 @@ function dimmatch end
 `Size` is used extensively throughout the `StaticArrays` API to describe _compile-time_
 knowledge of the size of an array. The dimensions are stored as a type parameter and are
 statically propagated by the compiler, resulting in efficient, type-inferrable code. For
-example, to create a static matrix of zeros, use `zeros(Size(3,3))` (rather than
-`zeros(3,3)`, which constructs a `Base.Array`).
+example, to create a static matrix of zeros, use `A = zeros(SMatrix{3,3})`. The static
+size of `A` can be obtained by `Size(A)`. (rather than `size(zeros(3,3))`, which returns
+`Base.Tuple{2,Int}`).
 
 Note that if dimensions are not known statically (e.g., for standard `Array`s),
 [`Dynamic()`](@ref) should be used instead of an `Int`.