Documentation: kdump: describe VMCOREINFO export for RISCV64

The following interrelated definitions and ranges are needed by the
kdump crash tool, which are exported by
"arch/riscv/kernel/crash_core.c":

    VA_BITS,
    PAGE_OFFSET,
    phys_ram_base,
    KERNEL_LINK_ADDR,
    MODULES_VADDR ~ MODULES_END,
    VMALLOC_START ~ VMALLOC_END,
    VMEMMAP_START ~ VMEMMAP_END,

Document these RISCV64 exports above.

Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Xianting Tian <xianting.tian@linux.alibaba.com>
Acked-by: Baoquan He <bhe@redhat.com>
Link: https://lore.kernel.org/r/20221026144208.373504-3-xianting.tian@linux.alibaba.com


[Palmer: wrap commit text]
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>