efi/libstub: Describe efi_relocate_kernel()

Update the description of of efi_relocate_kernel() to match Sphinx style.

Update parameter references in the description of other memory functions
to use @param style.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20200220065317.9096-1-xypron.glpk@gmx.de
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>