[atomics.syn] Missing `// optional` comments for `atomic_intN_t` aliases

[eisenwave]

These are simply declared in [atomics.syn] as

  using atomic_int8_t         = atomic<int8_t>;
  using atomic_uint8_t        = atomic<uint8_t>;
  using atomic_int16_t        = atomic<int16_t>;
  using atomic_uint16_t       = atomic<uint16_t>;
  using atomic_int32_t        = atomic<int32_t>;
  using atomic_uint32_t       = atomic<uint32_t>;
  using atomic_int64_t        = atomic<int64_t>;
  using atomic_uint64_t       = atomic<uint64_t>;

but [cstdint.syn] comes with comments, like:

  using uint8_t        = unsigned integer type; // optional
  using uint16_t       = unsigned integer type; // optional
  using uint32_t       = unsigned integer type; // optional
  using uint64_t       = unsigned integer type; // optional
  using uintN_t        = see below;             // optional

[Halalaluyafail3]

[stdatomic.h.syn] should be updated in the same way as well.