Merge bitcoin#22892: doc: Clarify that change_cost cannot be negative in GetSelectionWaste

d2eccac doc: Clarify that change_cost cannot be negative in GetSelectionWaste (benthecarman)

Pull request description:

  We assert that the `change_cost` must be positive so we should document it in the function's docs

  https://github.com/bitcoin/bitcoin/blob/4f5ad43b1e05cd7b403f87aae4c4d42e5aea810b/src/wallet/coinselection.cpp#L361

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Tree-SHA512: 089bc41d7e212b811455527ce2ac83301eae0edc03933dcbd5229b22078827395c8e3bcfda0ba029b7c60705db3119b4f80e372842a6bd8dae195dbaa0b52833

Author: laanwj

src/wallet/coinselection.h

@@ -174,7 +174,9 @@ struct OutputGroup
  * change_cost = effective_feerate * change_output_size + long_term_feerate * change_spend_size
  *
  * @param[in] inputs The selected inputs
- * @param[in] change_cost The cost of creating change and spending it in the future. Only used if there is change. Must be 0 if there is no change.
+ * @param[in] change_cost The cost of creating change and spending it in the future.
+ *                        Only used if there is change, in which case it must be positive.
+ *                        Must be 0 if there is no change.
  * @param[in] target The amount targeted by the coin selection algorithm.
  * @param[in] use_effective_value Whether to use the input's effective value (when true) or the real value (when false).
  * @return The waste