Documentation/no_hz: Introduce "dyntick-idle mode" before using it

The CONFIG_NO_HZ_IDLE paragraph is using "dyntick-idle mode" before
having defined it while the definition comes a couple of paragraphs
later.

That is leaving the reader with scratching head what that dyntick-idle
mode might be. Pull its definition up so that it is clear.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20210914203355.21360-1-bp@alien8.de
Signed-off-by: Jonathan Corbet <corbet@lwn.net>