x86/boot/compressed/64: Explain paging_prepare()'s return value

paging_prepare() returns a two-quadword structure which lands
into RDX:RAX:

  - Address of the trampoline is returned in RAX.
  - Non zero RDX means trampoline needs to enable 5-level paging.

Document that explicitly.

Signed-off-by: Kirill A. Shutemov <[email protected]>
Signed-off-by: Borislav Petkov <[email protected]>
Cc: [email protected]
Cc: "H. Peter Anvin" <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Kyle D Pelton <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Wei Huang <[email protected]>
Cc: x86-ml <[email protected]>
Link: https://lkml.kernel.org/r/[email protected]

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