Add sentence about distribution objects (#1565)

* Add sentence about distribution objects

* Apply suggestions from code review

Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Co-authored-by: David Widmann <devmotion@users.noreply.github.com>

Author: mschauer

docs/src/starting.md

@@ -22,7 +22,18 @@ Then, we create a standard-normal distribution `d` and obtain samples using `ran
 ```julia
 julia> d = Normal()
 Normal(μ=0.0, σ=1.0)
+```
+
+The object `d` represents a probability distribution, in our case the standard-normal distribution.
+One can query its properties such as the mean:
+
+```julia
+julia> mean(d)
+0.0
+```
 
+We can also draw samples from `d` with `rand`.
+```julia
 julia> x = rand(d, 100)
 100-element Array{Float64,1}:
   0.376264