add doc for divergence

src/common.jl

@@ -76,6 +76,16 @@ end
     end
 end
 
+"""
+    divergence(w::StaticKernels.Window)
+
+Kernel function to compute the divergence of a vector field.
+The window is expected to have eltype either:
+  - `SVector{N,T}` and `N` dimensions or
+  - `SMatrix{M,N,T}` and `N` dimensions.
+"""
+function divergence end
+
 @generated function divergence(w::StaticKernels.Window{SVector{N,T},N}) where {N,T}
     i0 = ntuple(_->0, N)
     i1(k) = ntuple(i->Int(k==i), N)