Documentation/x86: explain LINUX_EFI_INITRD_MEDIA_GUID

Documentation/x86: explain LINUX_EFI_INITRD_MEDIA_GUID

Since the Handover Protocol was deprecated, the recommended approach is
to provide an initrd using a UEFI boot service with the
LINUX_EFI_INITRD_MEDIA_GUID device path. Documentation for the new
approach has been no more than an admonition with a link to an existing
implementation.

Provide a short explanation of this functionality, to ease future
implementations without having to reverse engineer existing ones.

[Bagas: Don't use :ref: link to EFI stub documentation and refer to
OVMF/edk2 implementation]

Signed-off-by: Hugo Osvaldo Barrera <hugo@whynothugo.nl>
Link: https://lore.kernel.org/r/20250428131206.8656-2-hugo@whynothugo.nl
Co-developed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Message-ID: <20251013085718.27085-1-bagasdotme@gmail.com>