Merge pull request #271 from d-v-b/motivate_c

spec: add a motivation for the c prefix on chunk keys

Author: jbms

docs/v3/core/v3.0.rst

@@ -529,7 +529,8 @@ mandatory names:
 
       For example, in a 3 dimensional array, with a separator of ``/`` the identifier
       for the chunk at grid index (1, 23, 45) is the string ``"c/1/23/45"``.  With a
-      separator of ``.``, the identifier is the string ``"c.1.23.45"``.
+      separator of ``.``, the identifier is the string ``"c.1.23.45"``. The initial prefix 
+      ``c`` ensures that metadata documents and chunks have separate prefixes.
 
       .. note:: A main difference with spec v2 is that the default chunk separator
          changed from ``.`` to ``/``, as in N5.  This decreases the maximum number of