Documentation: sysrq: fix RST formatting

"On x86" and "On SPARC" are now definition list terms, like
"On PowerPC", "On other", and "On all".

The Credits list is now a bulleted list, like lots of Credits lists in
other files.  This prevents the list from becoming a single long,
unpunctuated sentence in the generated documentation.

I also did a couple of other tiny readability improvements to the
"How do I use the magic SysRq key?" section while I was there.

Signed-off-by: Alyssa Ross <hi@alyssa.is>
Link: https://lore.kernel.org/r/20200403170701.10852-1-hi@alyssa.is


Signed-off-by: Jonathan Corbet <corbet@lwn.net>