Documentation/namespace-description: Clarify label-less restrictions

In the ndctl-create-namespace (and related) man pages, add a
clarification note regarding some of the restrictions a user may see
when operating on label-less namespaces.

Link: #52
Reported-by: Jane Chu <jane.chu@oracle.com>
Cc: Dan Williams <dan.j.williams@intel.com>
Reviewed-by: Jeff Moyer <jmoyer@redhat.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>

Documentation/ndctl/namespace-description.txt

@@ -18,6 +18,15 @@ the kernel's 'memmap=ss!nn' command line option (see the nvdimm wiki on
 kernel.org), or NVDIMMs without a valid 'namespace index' in their label
 area.
 
+NOTE: Label-less namespaces lack many of the features of their label-rich
+cousins. For example, their size cannot be modified, or they cannot be
+fully 'destroyed' (i.e. the space reclaimed). A destroy operation will
+zero any mode-specific metadata. Finally, for create-namespace operations
+on label-less namespaces, ndctl bypasses the region capacity availability
+checks, and always satisfies the request using the full region capacity.
+The only reconfiguration operation supported on a label-less namespace
+is changing its 'mode'.
+
 A namespace can be provisioned to operate in one of 4 modes, 'fsdax',
 'devdax', 'sector', and 'raw'. Here are the expected usage models for
 these modes: