Remove 'fc'; add a 'block' protocol value

Change-Id: I9204cbaa61733fd2c76198e9816baee5fb9cf611
diff --git a/config.yaml b/config.yaml
index 49b2cdb..9c93045 100644
--- a/config.yaml
+++ b/config.yaml
@@ -26,7 +26,9 @@
     type: string
     default: iscsi
     description: |
-      SAN protocol to use. Choose between iscsi or fc.
+      The only supported value is "iscsi"; there may be experimental
+      support for the StorPool block client specified by "block" in the
+      future.
   volume-backend-name:
     type: string
     description: |