Doc: add doc to state that the --log option is deprecated

Since kernel(>=v5.10), a new lockless ringbuffer is introduced. And
crash commit a5531b2 ("printk: add support for lockless ringbuffer")
implemented the lockless ringbuffer dumping for cmd_log, this however,
relies on the existence of kernel debuginfo.

Since there is already a similar function been implemented for
makedumpfile, aka "makedumpfile --dump-dmesg" to dump dmesg logs with
only vmcore, there is no need to maintain a similar code in crash
as well. In addition, this option is not widely used, so just state the
"--log" option is deprecated.

Signed-off-by: Tao Liu <[email protected]>

crash.8

@@ -337,6 +337,8 @@ devices.
 Dump the contents of the kernel log buffer.  A kernel namelist
 argument is not necessary, but the dumpfile must contain the
 VMCOREINFO data taken from the original /proc/vmcore ELF header.
+Note: this option is deprecated and will no longer work for
+kernel(>=v5.10).
 .TP
 .B --no_kallsyms
 Do not use kallsyms-generated symbol information contained within 

help.c

@@ -215,6 +215,8 @@ char *program_usage_info[] = {
     "    Dump the contents of the kernel log buffer.  A kernel namelist",
     "    argument is not necessary, but the dumpfile must contain the",
     "    VMCOREINFO data taken from the original /proc/vmcore ELF header.",
+    "    Note: this option is deprecated and will no longer work for",
+    "    kernel(>=v5.10).",
     "",
     "  --no_kallsyms",
     "    Do not use kallsyms-generated symbol information contained within",