Documentation/magic-number.txt: convert it to ReST markup

- add a title for the document;
- convert the table;
- use quote block for the changelog;
- use monotonic fonts for file names;
- adjust whitespaces and blank lines;
- add it to the user's book.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>