Embedded Linux: explain what happens with an initramfs without /init

Update the "booting with initramfs" diagram.
Linux falls back to mounting the root device specified by root=
Tested on ARM on Linux 5.10

Signed-off-by: Michael Opdenacker <michael.opdenacker@bootlin.com>

slides/boot-sequence-initramfs/initramfs-boot-sequence.dia

@@ -87,13 +87,13 @@
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@@ -303,7 +303,7 @@ Mounts the root filesystem and switches to it (switch_root)
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@@ -422,14 +422,15 @@ Loads DTB + kernel to RAM, starts the kernel#</dia:string>
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             <dia:string>#Initializes hardware devices and kernel subsystems
 Extracts the initramfs archive to the file cache
-Starts the /init executable if found#</dia:string>
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+(otherwise falls back to mounting the device specified by root=)#</dia:string>
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