[3.13] gh-146227: Fix wrong type in _Py_atomic_load_uint16 in pyatomic_std.h (gh-146229) (#146233)

Also fix a related issue in the pyatomic headers:

* Fix pseudo-code comment for _Py_atomic_store_ptr_release in
  pyatomic.h.

(cherry picked from commit 1eff27f2c0452b3114bcf139062c87c025842c3e)