change docstr for vp-pdffromreplicas

validphys2/src/validphys/scripts/vp_pdffromreplicas.py

@@ -11,13 +11,11 @@ source ``PDF`` will be downloaded if it cannot be found locally.
 This script will not overwrite any existing files, so a ``PDF`` cannot already
 exist with the same name as the output ``PDF``.
 
-Note that whilst in principle it is possible to create a single replica ``PDF``
-whose replica 0 is simply the same as replica 1, LHAPDF has a restriction which
-requires ``PDF`` s to have at least 2 replicas (plus replica 0). To handle this
-special case if ``replicas == 1``, then replica 2 will be a duplicate of replica
+Note that whilst in principle it is possible to create a single replica ``PDF``,
+some applications might rely on the existence of 2 members (in addition to replica 0).
+To handle this special case if ``replicas == 1``, then replica 2 will be a duplicate of replica
 1, satisfying the minimum number of replicas whilst retaining the property
 that replica 1 and replica 0 are identical.
-
 """
 
 import argparse