docs: ABI: sysfs-class-firmware-attributes: solve some warnings

The Description: tag is missing on some places, causing
scripts/get_abi.pl warnings:

	Warning: file Documentation/ABI/testing/sysfs-class-firmware-attributes#172:
		What '/sys/class/firmware-attributes/*/authentication/' doesn't have a description

Also, some warnings are produced when generating html documentation:

	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:2: WARNING: Title underline too short.

	Dell specific class extensions
	--------------------------
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:2: WARNING: Unexpected indentation.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:2: WARNING: Unexpected indentation.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:2: WARNING: Block quote ends without a blank line; unexpected unindent.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:173: WARNING: Unexpected indentation.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:173: WARNING: Unexpected indentation.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:173: WARNING: Block quote ends without a blank line; unexpected unindent.
	.../Documentation/ABI/testing/sysfs-class-firmware-attributes:111: WARNING: Inline emphasis start-string without end-string.

Address the warnings, making it to produce the expected
output for the documentation ABI.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/44b4181b4f772fcc5ec676e72b295c10df35121b.1603963862.git.mchehab+huawei@kernel.org


Signed-off-by: Hans de Goede <hdegoede@redhat.com>