TL;DR: Document why exit_boot_services needs to do more than just
       call the UEFI function.

Exiting UEFI boot services requires a non-trivial sequence of steps,
including safe retrieval and finalization of the memory map. To clarify
why our exit_boot_services function is more than a thin wrapper, the
documentation has been updated accordingly.

This complexity is also reflected in the Linux source code [0].
In fact, the sequence of steps uefi-rs uses is inspired by the Linux
source code.

[0] https://.com/torvalds/linux/blob/e544a0743/drivers/firmware/efi/libstub/efi-stub-helper.c#L375

People probably want to use 99% of the time LOADER_DATA here.
Therefore, we indicate in our function signature, that people should
use the default (`None`) if they are unsure what to do here.

LOADER_DATA is also what Linux uses [0].

[0] https://elixir.bootlin.com/linux/v6.13.7/source/drivers/firmware/efi/libstub/mem.c#L24