Documentation/misc-devices/mei: Fix formatting of headings.

Make the heading underlines fit the length of the heading, remove colons
at the end of headings and consistently place an empty line after each
heading and two empty lines before each that is preceded by a paragraph.

Reviewed-by: Jeremiah Mahler <jmmahler@gmail.com>
Signed-off-by: Julian Brost <linux-kernel@0x4a42.net>
Signed-off-by: Fabian Hofmann <fabian.hofmann@fau.de>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>