Improves documentation on `num_slashing_spans` when calling...

Improves documentation on `num_slashing_spans` when calling `withdraw_unbounded` in Staking. (#14185)

* gpestana/11714-num_slashing_spans_docs

* Update frame/staking/src/pallet/mod.rs

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>

* All calls that take num_slashing_span as parameters refer to comments on it

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Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>