bluestore-wal only needed if separate to DB device

Update config.yaml to clarify that bluestore-wal should only be set
where a separate (faster) device is being used for the WAL. Otherwise,
the WAL is automatically maintained within the space of the DB device
and does not need to be configured separately.

Additionally clarify that this device is used as an LVM PV and space is
allocated for each block device based on the
bluestore-block-{db,wal}-size setting.

Change-Id: I54fc582ecb2cee5de1302685e9103c636c7a307b
Ref: http://docs.ceph.com/docs/master/rados/configuration/bluestore-config-ref/

config.yaml

@@ -85,12 +85,24 @@ options:
     type: string
     default:
     description: |
-      Path to a BlueStore WAL block device or file.
+      Path to a BlueStore WAL block device or file. Should only be set if using
+      a separate physical device that is faster than the DB device (such as an
+      NVDIMM or faster SSD). Otherwise BlueStore automatically maintains the
+      WAL inside of the DB device. This block device is used as an LVM PV and
+      then space is allocated for each block device as needed based on the
+      bluestore-block-wal-size setting.
   bluestore-db:
     type: string
     default:
     description: |
-      Path to a BlueStore WAL db block device or file
+      Path to a BlueStore WAL db block device or file. If you have a separate
+      physical device faster than the block device this will store all of the
+      filesystem metadata (RocksDB) there and also integrates the Write Ahead
+      Log (WAL) unless a further separate bluestore-wal device is configured
+      which is not needed unless it is faster again than the bluestore-db
+      device. This block device is used as an LVM PV and then space is
+      allocated for each block device as needed based on the
+      bluestore-block-db-size setting.
   osd-journal-size:
     type: int
     default: 1024