docs: fault-injection: document cache-filter feature for failslab

The failslab fault injection mechanism has an undocumented capability
that provides significant utility in testing and debugging. This feature,
introduced in commit 4c13dd3b48fcb ("failslab: add ability to filter slab
caches"), allows for targeted error injection into specific slab caches.

However, it was inadvertently left undocumented at the time of its
implementation.

Add documentation for the cache-filter feature in the failslab mode
description. Also, providing a practical example demonstrating how to
use cache-filter to inject failures specifically when allocating socket
buffers (skbs).

Signed-off-by: Breno Leitao <[email protected]>
Reviewed-by: Akinobu Mita <[email protected]>
Signed-off-by: Jonathan Corbet <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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