Documentation: gpio: pca953x: clarify interrupt source detection

There are multiple design tradeoffs and considerations in how the
PCA953x driver detects the source(s) of an interrupt. This driver
supports PCAL variants with input latching, a feature that is
constrained by the fact that the interrupt status and input port
registers cannot be read atomically. These limits and the design
decisions deserve an in-depth explanation.

Update the documentation to clarify these hardware limits and describe
how the driver determines pending interrupts, and how it makes use of
the PCAL input latching.

Signed-off-by: Ernest Van Hoecke <ernest.vanhoecke@toradex.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20260107093125.4053468-1-ernestvanhoecke@gmail.com
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>

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