Clarify the non-SPV_SUCCESS case parser callbacks.
Clarify the non-SPV_SUCCESS case parser callbacks.

When a callback returns something other than SPV_SUCCESS,
then no futher callbacks are issued (parsing terminates early),
and no additional diagnostics are emitted.

The unit tests already check this behaviour.

pierremoreau/SPIRV-Tools,umar456/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,KhronosGroup/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,dneto0/SPIRV-Tools,umar456/SPIRV-Tools,greg-lunarg/SPIRV-Tools,antiagainst/SPIRV-Tools,antiagainst/SPIRV-Tools,dneto0/SPIRV-Tools,antiagainst/SPIRV-Tools,KhronosGroup/SPIRV-Tools,dneto0/SPIRV-Tools,greg-lunarg/SPIRV-Tools,KhronosGroup/SPIRV-Tools,umar456/SPIRV-Tools,LukasBanana/SPIRV-Tools,KhronosGroup/SPIRV-Tools,antiagainst/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,LukasBanana/SPIRV-Tools,pierremoreau/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,KhronosGroup/SPIRV-Tools,greg-lunarg/SPIRV-Tools,fuchsia-mirror/third_party-spirv-tools,pierremoreau/SPIRV-Tools,LukasBanana/SPIRV-Tools,dneto0/SPIRV-Tools,pierremoreau/SPIRV-Tools,pierremoreau/SPIRV-Tools,antiagainst/SPIRV-Tools,KhronosGroup/SPIRV-Tools