tracing, doc: update mmiotrace documentation

Impact: update documentation

Update to reflect the current state of the tracing framework:

 - "none" tracer has been replaced by "nop" tracer
 - tracing_enabled must be toggled when changing buffer size

Signed-off-by: Pekka Paalanen <pq@iki.fi>
Signed-off-by: Ingo Molnar <mingo@elte.hu>