doc: fix crush-ruleset-name param description

Specified crush-ruleset-name is required to exist, implicit creation is
going to happen only if crush-ruleset-name wasn't specified on the
command line.  While at it, pool-name is very much a required param.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

doc/rados/configuration/pool-pg-config-ref.rst

@@ -70,7 +70,9 @@ Ceph configuration file.
 
 :Description: The default CRUSH ruleset to use when creating a replicated pool.
 :Type: 8-bit Integer
-:Default: ``0``
+:Default: ``CEPH_DEFAULT_CRUSH_REPLICATED_RULESET``, which means "pick
+          a ruleset with the lowest numerical ID and use that".  This is to
+          make pool creation work in the absence of ruleset 0.
 
 
 ``osd pool erasure code stripe width`` 

doc/rados/operations/pools.rst

@@ -71,7 +71,7 @@ Where:
 
 :Description: The name of the pool. It must be unique.
 :Type: String
-:Required: Yes. Picks up default or Ceph configuration value if not specified.
+:Required: Yes.
 
 ``{pg-num}``
 
@@ -110,15 +110,17 @@ Where:
 
 ``[crush-ruleset-name]``
 
-:Description: The name of the crush ruleset for this pool. If specified ruleset
-              doesn't exist, the creation of **replicated** pool will fail with
-              -ENOENT. But **replicated** pool will create a new erasure 
-              ruleset with specified name.
+:Description: The name of a CRUSH ruleset to use for this pool.  The specified
+              ruleset must exist.
 
 :Type: String
 :Required: No. 
-:Default: "erasure-code" for **erasure pool**. Pick up Ceph configuraion variable
-          **osd_pool_default_crush_replicated_ruleset** for **replicated** pool.
+:Default: For **replicated** pools it is the ruleset specified by the ``osd
+          pool default crush replicated ruleset`` config variable.  This
+          ruleset must exist.
+          For **erasure** pools it is ``erasure-code`` if the ``default``
+          `erasure code profile`_ is used or ``{pool-name}`` otherwise.  This
+          ruleset will be created implicitly if it doesn't exist already.
 
 
 ``[erasure-code-profile=profile]``