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

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

Compare the atomic_intN_t aliases in atomics.syn with the corresponding declarations and comments in cstdint.syn. Update the aliases so the optional status is represented consistently, then verify the generated draft preserves the intended comments.