Documentation: riscv: uabi: Clarify ISA spec version for canonical order

Documentation: riscv: uabi: Clarify ISA spec version for canonical order

Specify that chapter 27 refers to version 20191213 of the RISC-V ISA
Unprivileged Architecture. The chapter numbering differs across
specification versions - for example, in version 20250508, the ISA
Extension Naming Conventions is chapter 36, not chapter 27.

Historical versions of the RISC-V specification can be found via Link [1].

Acked-by: Paul Walmsley <pjw@kernel.org>
Link: https://riscv.org/specifications/ratified/
Fixes: f07b2b3f9d47 ("Documentation: riscv: add a section about ISA string ordering in /proc/cpuinfo")
Signed-off-by: Guodong Xu <guodong@riscstar.com>
Link: https://patch.msgid.link/20260115-adding-b-dtsi-v2-1-254dd61cf947@riscstar.com