Improves the early-kdump-howto.txt document in several points:

(1) explain early kdump a little clearer in "Introduction"
(2) move the notes out to a new "Notes" section for readability and
    add a note about need of reconfiguration after kernel update
(3) change journalctl -x option to -b option because -x is unnecessary
    and -b will make it very faster if persistent journal is available
(4) shorten the example messages for readability
(5) add a note to "Limitation" about the earliness of early kdump

Signed-off-by: Kazuhito Hagio <k-hagio-ab@nec.com>
Acked-by: Lianbo Jiang <lijiang@redhat.com>

early-kdump-howto.txt

@@ -3,12 +3,17 @@ Early Kdump HOWTO
 Introduction
 ------------
 
-Kdump service starts too late, so early crashes will have no chance to get
-kdump kernel booting, this will cause crash information to be lost. It is
-necessary to add a dracut module in order to load crash kernel and initramfs
-as early as possible. You can provide "rd.earlykdump" in grub commandline
-to enable, then the early kdump will load those files like the normal kdump,
-which is disabled by default.
+Early kdump is a mechanism to make kdump operational earlier than normal kdump
+service.  The kdump service starts early enough for general crash cases, but
+there are some cases where it has no chance to make kdump operational in boot
+sequence, such as detecting devices and starting early services.  If you hit
+such a case, early kdump may allow you to get more information of it.
+
+Early kdump is implemented as a dracut module.  It adds a kernel (vmlinuz) and
+initramfs for kdump to your system's initramfs in order to load them as early
+as possible.  After that, if you provide "rd.earlykdump" in kernel command line,
+then in the initramfs, early kdump will load those files like the normal kdump
+service.  This is disabled by default.
 
 For the normal kdump service, it can check whether the early kdump has loaded
 the crash kernel and initramfs. It has no conflict with the early kdump.
@@ -20,7 +25,9 @@ We assume if you're reading this document, you should already have kexec-tools
 installed.
 
 You can rebuild the initramfs with earlykdump support with below steps:
+
 1. start kdump service to make sure kdump initramfs is created.
+
     # systemctl start kdump
 
     NOTE: If a crash occurs during boot process, early kdump captures a vmcore
@@ -34,6 +41,7 @@ You can rebuild the initramfs with earlykdump support with below steps:
     For the failure_action, you can choose anything other than "reboot".
 
 2. rebuild system initramfs with earlykdump support.
+
     # dracut --force --add earlykdump
 
     NOTE: Recommend to backup the original system initramfs before performing
@@ -41,16 +49,6 @@ You can rebuild the initramfs with earlykdump support with below steps:
 
 3. add rd.earlykdump in grub kernel command line.
 
-Note:
-[1]. Early kdump initramfs size will be large because it includes vmlinuz and
-kdump initramfs.
-
-[2]. Early kdump inherits the settings of normal kdump, so any changes that
-caused normal kdump rebuilding also require rebuilding the system initramfs
-to make sure that the changes take effect for early kdump. Therefore, after
-the rebuilding of kdump initramfs is completed, provide a prompt message to
-tell the fact.
-
 After making said changes, reboot your system to take effect. Of course, if you
 want to disable early kdump, you can simply remove "rd.earlykdump" from kernel
 boot parameters in grub, and reboot system like above.
@@ -58,19 +56,40 @@ boot parameters in grub, and reboot system like above.
 Once the boot is completed, you can check the status of the early kdump support
 on the command prompt:
 
-    # journalctl -x|grep early-kdump
+    # journalctl -b | grep early-kdump
 
 Then, you will see some useful logs, for example:
 
-1. if early kdump is successful.
-Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: early-kdump is enabled.
-Mar 09 09:57:56 localhost.localdomain dracut-cmdline[190]: kexec: loaded early-
-kdump kernel
+- if early kdump is successful.
 
-2. if early kdump is disabled.
-Mar 09 10:02:47 localhost.localdomain dracut-cmdline[189]: early-kdump is disabled.
+Mar 09 09:57:56 localhost dracut-cmdline[190]: early-kdump is enabled.
+Mar 09 09:57:56 localhost dracut-cmdline[190]: kexec: loaded early-kdump kernel
+
+- if early kdump is disabled.
+
+Mar 09 10:02:47 localhost dracut-cmdline[189]: early-kdump is disabled.
+
+Notes
+-----
+
+- The size of early kdump initramfs will be large because it includes vmlinuz
+  and kdump initramfs.
+
+- Early kdump inherits the settings of normal kdump, so any changes that
+  caused normal kdump rebuilding also require rebuilding the system initramfs
+  to make sure that the changes take effect for early kdump. Therefore, after
+  the rebuilding of kdump initramfs is completed, provide a prompt message to
+  tell the fact.
+
+- If you install an updated kernel and reboot the system with it, the early
+  kdump will be disabled by default.  To enable it with the new kernel, you
+  need to take the above steps again.
 
 Limitation
 ----------
 
-At present, early kdump doesn't support fadump.
+- At present, early kdump doesn't support fadump.
+
+- Early kdump loads a crash kernel and initramfs at the beginning of the
+  process in system's initramfs, so a crash at earlier than that (e.g. in
+  kernel initialization) cannot be captured even with the early kdump.