Documentation/sysrq.txt: convert to ReST markup

- Fix document title;
- use a table for the valid commands;
- use quote blocks where needed;
- use monotonic fonts for config options and file names;
- adjust whitespaces and blank lines;
- add it to the user's book.
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>