[AArch64][Docs] Release notes 12.x on outline atomics

[ilinpv]

Summary

Description for AArch64 -moutline-atomics, -mno-outline-atomics
options added to release notes.

[ilinpv]

Fixing typos

[t.p.northover]

I think this is fine.

clang/docs/ReleaseNotes.rst

82

Feel free to ignore since they're so minor, but if you're looking for more typos to fix:

• The comma after "do not apply" is unnecessary.
• "the compilation target" would be a bit more natural.
• Missing apostrophe in "option's behaviour".

No need to reupload to Phab as far as I'm concerned either way.

[ilinpv]

Tim, thank you for review, noticed typos and overall support to outline atomics patches!
Notes pushed rG98f06b16a313ece593f5711778d7da9037f3a2ef

clang/docs/ReleaseNotes.rst

- The builtin intrinsics ``__builtin_rotateright8``, ``__builtin_rotateright16``,
   ``__builtin_rotateright32`` and ``__builtin_rotateright64`` may now be used
   within constant expressions.
 
 New Compiler Flags
 ------------------
 
 - ...
 
+- AArch64 options ``-moutline-atomics``, ``-mno-outline-atomics`` to enable
+  and disable calls to helper functions implementing atomic operations. These
+  out-of-line helpers like '__aarch64_cas8_relax' will detect at runtime
+  AArch64 Large System Extensions (LSE) availability and either use their
+  atomic instructions, or fall back to LL/SC loop. These options do not apply,
+  if compilation target supports LSE. Atomic instructions are used directly in
+  that case. The options behaviour mirrors GCC, the helpers are implemented
+  both in compiler-rt and libgcc.
+
 - -fpch-codegen and -fpch-debuginfo generate shared code and/or debuginfo
   for contents of a precompiled header in a separate object file. This object
   file needs to be linked in, but its contents do not need to be generated
   for other objects using the precompiled header. This should usually save
   compile time. If not using clang-cl, the separate object file needs to
   be created explicitly from the precompiled header.
   Example of use: