l2arc_exclude_special not detailed in relevant documentation page

[kewiha]

The Module Parameters page in the Performance and Tuning Section here does not detail the relatively new parameter "l2arc_exclude_special", but arguably it's relevant to performance and tuning.

The l2arc_exclude_special parameter is detailed in here and so most of the information could be snagged from there.

I can fork and propose edits if you want, but it may be less effort for a reviewer to make the change themselves rather than review my attempt.

[gmelikov]

Contributions are welcome, I'd be glad to review your patch.

[gmelikov]

Closed via https://github.com/openzfs/openzfs-docs/pull/275