[SYCL][Doc] Add OpDeviceRegistryVersionINTEL; Misc changes

[jzadnik]

This PR adds OpDeviceRegistryVersionINTEL to specify which version of the device registry was used when producing a module. This allows a consumer to correctly interpret OpSpecConstantArchitectureINTEL and OpSpecConstantTargetINTEL instructions without any knowledge about the producer. Currently, the producer implemented in SPIRV-Tools adds an OpModuleProcessed note with the version, but since the version carries a semantic information (ie. determines the result of the OpSpecConstantArchitecture/TargetINTEL instructions), I think it should be a separate, non-debug, instruction.

Other changes:

• Rename the "targets registry" to "device registry" to disambiguate from the "target" used in OpSpecConstantTargetINTEL.
• Allow annotating OpExtInst with ConditionalINTEL because it can be in the types section outside a function in the case of non-semantic instructions.
• Update to SPIRV spec. version 1.6 rev. 6 which changes the numbering.
• Misc wording and cosmetic changes.

To be merged after #19782