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/#mf24d7070d7e0c8f17b6be6ceb51df94b7d7613b3
Reviewed-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.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>