bootconfig: Fix Kconfig help message for BOOT_CONFIG

Fix Kconfig help message since the bootconfig file is
only available to be appended to initramfs. And also
add a reference to the documentation.

Link: http://lkml.kernel.org/r/157949058031.25888.18399447161895787505.stgit@devnote2Reported-by: NRandy Dunlap <rdunlap@infradead.org>
Acked-by: NRandy Dunlap <rdunlap@infradead.org>
Signed-off-by: NMasami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: NSteven Rostedt (VMware) <rostedt@goodmis.org>

init/Kconfig

@@ -1223,7 +1223,9 @@ config BOOT_CONFIG
 	help
 	  Extra boot config allows system admin to pass a config file as
 	  complemental extension of kernel cmdline when booting.
-	  The boot config file is usually attached at the end of initramfs.
+	  The boot config file must be attached at the end of initramfs
+	  with checksum and size.
+	  See <file:Documentation/admin-guide/bootconfig.rst> for details.
 
 	  If unsure, say Y.