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>

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -595,3 +595,32 @@ X2TLB
 -----
 
 Indicates whether the crashed kernel enabled SH extended mode.
+
+RISCV64
+=======
+
+VA_BITS
+-------
+
+The maximum number of bits for virtual addresses. Used to compute the
+virtual memory ranges.
+
+PAGE_OFFSET
+-----------
+
+Indicates the virtual kernel start address of the direct-mapped RAM region.
+
+phys_ram_base
+-------------
+
+Indicates the start physical RAM address.
+
+MODULES_VADDR|MODULES_END|VMALLOC_START|VMALLOC_END|VMEMMAP_START|VMEMMAP_END|KERNEL_LINK_ADDR
+----------------------------------------------------------------------------------------------
+
+Used to get the correct ranges:
+
+  * MODULES_VADDR ~ MODULES_END : Kernel module space.
+  * 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