update docs

docs/source/builders/random.rst

@@ -53,6 +53,7 @@ more concentrated.
     composition: 
       Cu: 13
       H2O: 3
+    # pbc: True # Set this to True if periodic structures are desired.
     cell: [30., 0., 0., 0., 30., 0., 0., 0., 30.]
     region:
       method: lattice
@@ -62,6 +63,12 @@ more concentrated.
     test_too_far: false
     random_seed: 1112
 
+.. note::
+
+   The structures generated by `random_cluster` are not periodic (pbc=False) by default. 
+   This may fail the following calculations as some codes do not support non-periodic structures.
+   Thus, set `pbc: True` to make generated structures periodic.
+
 Surface
 -------