Documentation: kdump: Add va_kernel_pa_offset for RISCV64

RISC-V Linux exports "va_kernel_pa_offset" in vmcoreinfo to help
Crash-utility translate the kernel virtual address correctly.

Here adds the definition of "va_kernel_pa_offset".

Fixes: 3335068f ("riscv: Use PUD/P4D/PGD pages for the linear mapping")
Link: https://lore.kernel.org/linux-riscv/20230724040649.220279-1-suagrfillet@gmail.com/Signed-off-by: Song Shuai <suagrfillet@gmail.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20230724100917.309061-2-suagrfillet@gmail.com
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -624,3 +624,9 @@ Used to get the correct ranges:
   * VMALLOC_START ~ VMALLOC_END : vmalloc() / ioremap() space.
   * VMEMMAP_START ~ VMEMMAP_END : vmemmap space, used for struct page array.
   * KERNEL_LINK_ADDR : start address of Kernel link and BPF
+
+va_kernel_pa_offset
+-------------------
+
+Indicates the offset between the kernel virtual and physical mappings.
+Used to translate virtual to physical addresses.