CKR_COPY_PROHIBITED - proposed text for section 8.1.6

[Michael StJohns <msj@nthpermutation.com>]

This is an editorial change that adds descriptive text for the 
CKR_COPY_PROHIBITED return code.  That return code was added in 2.30, 
but this documentation was omitted.

Insert in 8.1.6 after CKR_CANT_LOCK

CKR_COPY_PROHIBITED:  This value can only be returned by C_CopyObject.  
It indicates that the object may not be copied, either because of 
underlying policy restrictions on the token, or because the object has 
the CKA_COPYABLE attribute set to CK_FALSE.