kernel: doc: document wipe lifecycle and best entry nullability

Document on `btck_chainstate_manager_options_set_wipe_dbs` that a wipe must be followed by `btck_chainstate_manager_import_blocks` before the chainstate manager is used for anything else, as the existing kernel tests already do (e.g. `chainman_reindex_test`).

Note in the `@return` of `btck_chainstate_manager_get_best_entry` that it can return null when no block headers have been loaded.

Refactor the `@return` documentation to fit on a single line.

src/kernel/bitcoinkernel.h

@@ -1179,6 +1179,8 @@ BITCOINKERNEL_API void btck_chainstate_manager_options_set_worker_threads_num(
  * @param[in] wipe_block_tree_db         Set wipe block tree db. Should only be 1 if wipe_chainstate_db is 1 too.
  * @param[in] wipe_chainstate_db         Set wipe chainstate db.
  * @return                               0 if the set was successful, non-zero if the set failed.
+ * @note                                 When a wipe is set, the caller must invoke @ref btck_chainstate_manager_import_blocks
+ *                                       on the resulting chainstate manager before using it for anything else.
  */
 BITCOINKERNEL_API int BITCOINKERNEL_WARN_UNUSED_RESULT btck_chainstate_manager_options_set_wipe_dbs(
     btck_ChainstateManagerOptions* chainstate_manager_options,
@@ -1233,7 +1235,7 @@ BITCOINKERNEL_API btck_ChainstateManager* BITCOINKERNEL_WARN_UNUSED_RESULT btck_
  * known cumulative proof of work.
  *
  * @param[in] chainstate_manager Non-null.
- * @return                       The btck_BlockTreeEntry.
+ * @return                       The btck_BlockTreeEntry, or null if no block headers have been loaded.
  */
 BITCOINKERNEL_API const btck_BlockTreeEntry* BITCOINKERNEL_WARN_UNUSED_RESULT btck_chainstate_manager_get_best_entry(
     const btck_ChainstateManager* chainstate_manager) BITCOINKERNEL_ARG_NONNULL(1);