rearrange docstring of vectorized system

Describe more fundamental attributes first.

kwant/system.py

@@ -546,6 +546,18 @@ class VectorizedSystem(System, metaclass=abc.ABCMeta):
     ----------
     symmetry : kwant.system.Symmetry
         The symmetry of the system.
+    site_arrays : sequence of SiteArray
+        The sites of the system. The family of each site array must have
+        ``norbs`` specified.
+    site_ranges : Nx3 integer array
+        Has 1 row per site array, plus one extra row.  Each row consists
+        of ``(first_site, norbs, orb_offset)``: the index of the first
+        site in the site array, the number of orbitals on each site in
+        the site array, and the offset of the first orbital of the first
+        site in the site array.  In addition, the final row has the form
+        ``(len(graph.num_nodes), 0, tot_norbs)`` where ``tot_norbs`` is the
+        total number of orbitals in the system. Note 'site_ranges'
+        is directly computable from 'site_arrays'.
     graph : kwant.graph.CGraph
         The system graph.
     subgraphs : sequence of tuples
@@ -561,18 +573,6 @@ class VectorizedSystem(System, metaclass=abc.ABCMeta):
         'hermitian' is 'True' if the term needs its Hermitian
         conjugate to be added when evaluating the Hamiltonian, and 'parameters'
         contains a list of parameter names used when evaluating this term.
-    site_arrays : sequence of SiteArray
-        The sites of the system. The family of each site array must have
-        ``norbs`` specified.
-    site_ranges : Nx3 integer array
-        Has 1 row per site array, plus one extra row.  Each row consists
-        of ``(first_site, norbs, orb_offset)``: the index of the first
-        site in the site array, the number of orbitals on each site in
-        the site array, and the offset of the first orbital of the first
-        site in the site array.  In addition, the final row has the form
-        ``(len(graph.num_nodes), 0, tot_norbs)`` where ``tot_norbs`` is the
-        total number of orbitals in the system. Note 'site_ranges'
-        is directly computable from 'site_arrays'.
     parameters : frozenset of strings
         The names of the parameters on which the system depends. This attribute
         is provisional and may be changed in a future version of Kwant