RISC-V: Document the ISA string parsing rules for ACPI

We've had a ton of issues around the ISA string parsing rules elsewhere
in RISC-V, so let's at least be clear about what the rules are so we can
try and avoid more issues.

Link: https://lore.kernel.org/r/CAK9=C2Vy-4V9kgnga98tiC3TeHkR2LFPakyBbS8s_h3_Z=ieyQ@mail.gmail.com/
Link: https://lore.kernel.org/r/20230629031705.15575-1-palmer@rivosinc.comSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/riscv/acpi.rst

+.. SPDX-License-Identifier: GPL-2.0
+
+==============
+ACPI on RISC-V
+==============
+
+The ISA string parsing rules for ACPI are defined by `Version ASCIIDOC
+Conversion, 12/2022 of the RISC-V specifications, as defined by tag
+"riscv-isa-release-1239329-2023-05-23" (commit 1239329
+) <https://github.com/riscv/riscv-isa-manual/releases/tag/riscv-isa-release-1239329-2023-05-23>`_

Documentation/riscv/index.rst

@@ -5,6 +5,7 @@ RISC-V architecture
 .. toctree::
     :maxdepth: 1
 
+    acpi
     boot-image-header
     vm-layout
     hwprobe