Clarify ReadWriteOnce allows multiple pods to read and write

Courrently, description of `ReadWriteOnce` leaves room for misinterpretation
of the `access` in "multiple pods to access the volume" bit. This leads
to spread of confusing information like multiple pods can access but only
one pod can write at a time.

See also kubernetes/kubernetes#60903 (comment)

Author: mloskot

content/en/docs/concepts/storage/persistent-volumes.md

@@ -632,7 +632,7 @@ The access modes are:
 
 `ReadWriteOnce`
 : the volume can be mounted as read-write by a single node. ReadWriteOnce access
-  mode still can allow multiple pods to access the volume when the pods are
+  mode still can allow multiple pods to access, read or write, the volume when the pods are
   running on the same node. For single pod access, please see ReadWriteOncePod.
 
 `ReadOnlyMany`