doc/rados: pool and namespace are independent osdcap restrictions

doc/rados: pool and namespace are independent osdcap restrictions

For the "profile {name}" syntax, pool and namespace restrictions are
independent of each other (i.e. specifying namespace doesn't also
require specifying pool like is currently suggested).  A cap can look
like "profile rbd namespace=myns", signifying that the RBD profile is
to be allowed in myns namespace of any pool.

For the "allow {access-spec}" syntax, pool restriction is optional.
A cap can look like "allow r namespace=myns", "allow w object_prefix
myprefix" or "allow rw namespace=myns object_prefix myprefix", for
example.

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