lib: update DEBUG_SHIRQ docs to match reality

There is no extra interrupt when registering a shared interrupt handler
since 2011. Update the Kconfig text to make it clear and to avoid wrong
assumptions when debugging issues found by it.

Fixes: 6d83f94d ("genirq: Disable the SHIRQ_DEBUG call in request_threaded_irq for now")
Link: https://lore.kernel.org/linux-i2c/859e8211-2c56-8dd5-d6fb-33e4358e4128@pengutronix.de/T/#mf24d7070d7e0c8f17b6be6ceb51df94b7d7613b3Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20200702222024.6915-1-wsa+renesas@sang-engineering.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/Kconfig.debug

@@ -844,10 +844,10 @@ config DEBUG_SHIRQ
 	bool "Debug shared IRQ handlers"
 	depends on DEBUG_KERNEL
 	help
-	  Enable this to generate a spurious interrupt as soon as a shared
-	  interrupt handler is registered, and just before one is deregistered.
-	  Drivers ought to be able to handle interrupts coming in at those
-	  points; some don't and need to be caught.
+	  Enable this to generate a spurious interrupt just before a shared
+	  interrupt handler is deregistered (generating one when registering
+	  is currently disabled). Drivers need to handle this correctly. Some
+	  don't and need to be caught.
 
 menu "Debug Oops, Lockups and Hangs"