powerpc/fadump: update documentation about bootargs_append

Update ABI documentation about the introduction of the new sysfs
entry bootargs_append. This sysfs entry will be used to setup the
additional parameters to be passed to dump capture kernel.
Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240510082114.561163-1-hbathini@linux.ibm.com

Documentation/ABI/testing/sysfs-kernel-fadump

@@ -49,3 +49,10 @@ Description:	read only
 		memory add/remove events because elfcorehdr is now prepared in
 		the second/fadump kernel.
 User:		kexec-tools
+
+What:		/sys/kernel/fadump/bootargs_append
+Date:		May 2024
+Contact:	linuxppc-dev@lists.ozlabs.org
+Description:	read/write
+		This is a special sysfs file available to setup additional
+		parameters to be passed to capture kernel.