[sw/silicon_creator] Document usage constraints selector_bits granularity in the device identifier spec

Usage constraints `selector_bits` can be used to select individual words of the device identifier. However, the first two fields of the device identifier (silicon creator identifier and product identifier) are [16-bits](https://docs.opentitan.org/doc/security/specs/identities_and_root_keys/#device-identifier). We should mention that these fields will be treated as a single 32-bit field ([comment](https://github.com/lowRISC/opentitan/pull/7903#discussion_r696157686) in #7903).