efi/libstub: Descriptions for stub helper functions

Provide missing descriptions for EFI stub helper functions.
Adjust formatting of existing descriptions to kernel style.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Link: https://lore.kernel.org/r/20200615234231.21059-1-xypron.glpk@gmx.de


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>