improve cold boot attacks documentation

Originally created by Tails on #6008 (Redmine)

See cold boot attacks.

• Make it more concise and neutral.
• Make it more scannable with respect to:
  • emergency shutdown tools;
  • memory wipe procedure.
• Mention the limitations of memory wipe (that is? probably a duplicate of what’s already mentioned on known issues

Feature Branch: doc/cold-boot-attack