improve docstring

Author: CarloLucibello

src/layers/basic.jl

@@ -163,11 +163,22 @@ returns the dot product `x_i ⋅ xj` on each edge.
 # Usage 
 
 ```juliarepl
+julia> g = rand_graph(5, 6)
+GNNGraph:
+    num_nodes = 5
+    num_edges = 6
 
+julia> dotdec = DotDecoder()
+DotDecoder()
+
+julia> dotdec(g, rand(2, 5))
+1×6 Matrix{Float64}:
+ 0.345098  0.458305  0.106353  0.345098  0.458305  0.106353
 ```
 """
 struct DotDecoder <: GNNLayer end 
 
 function (::DotDecoder)(g, x)
+    check_num_nodes(g, x)
     apply_edges(xi_dot_xj, g, xi=x, xj=x)
 end