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

Documentation/timers/no_hz.rst

@@ -70,6 +70,10 @@ interrupt.  After all, the primary purpose of a scheduling-clock interrupt
 is to force a busy CPU to shift its attention among multiple duties,
 and an idle CPU has no duties to shift its attention among.
 
+An idle CPU that is not receiving scheduling-clock interrupts is said to
+be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
+tickless".  The remainder of this document will use "dyntick-idle mode".
+
 The CONFIG_NO_HZ_IDLE=y Kconfig option causes the kernel to avoid sending
 scheduling-clock interrupts to idle CPUs, which is critically important
 both to battery-powered devices and to highly virtualized mainframes.
@@ -91,10 +95,6 @@ Therefore, systems with aggressive real-time response constraints often
 run CONFIG_HZ_PERIODIC=y kernels (or CONFIG_NO_HZ=n for older kernels)
 in order to avoid degrading from-idle transition latencies.
 
-An idle CPU that is not receiving scheduling-clock interrupts is said to
-be "dyntick-idle", "in dyntick-idle mode", "in nohz mode", or "running
-tickless".  The remainder of this document will use "dyntick-idle mode".
-
 There is also a boot parameter "nohz=" that can be used to disable
 dyntick-idle mode in CONFIG_NO_HZ_IDLE=y kernels by specifying "nohz=off".
 By default, CONFIG_NO_HZ_IDLE=y kernels boot with "nohz=on", enabling