[AMDGPU] Update Memory Model in AMDGPUUsage.rst

[AMDGPU] Update Memory Model in AMDGPUUsage.rst

Mostly NFC, but some changes are "bug fixes" rather than just e.g.
formatting changes or typo corrections.

- Fix typo "competing" -> "completing".
- Document why waintcnt is added to stores and not loads for
  sequentially consistent ordering.
- Lowercase some mentions of `buffer_gl{0,1}_inv`.
- Make mentions of `*cnt(0)` consistently include the `(0)` count.
- Remove some mentions of instructions for incorrect address spaces. For
  example, remove mention of `flat_load` from
  `load atomic acquire workgroup global`.
- Re-flow some text to get all the target columns to fit in a
  32-character wide column. Makes a future NFC patch to make these columns
  both 32-character wide more straightforward.

Modified cherry-pick of patch by Tony Tye

Reviewed By: t-tye

Differential Revision: https://reviews.llvm.org/D89596