[doc]: add doxygen return comment for CheckPackageLimits

Author: ismaelsadeeq

src/txmempool.h

@@ -605,6 +605,7 @@ class CTxMemPool
      *                                          to mempool. The transactions need not be direct
      *                                          ancestors/descendants of each other.
      * @param[in]       total_vsize             Sum of virtual sizes for all transactions in package.
+     * @returns {} or the error reason if a limit is hit.
      */
     util::Result<void> CheckPackageLimits(const Package& package,
                                           int64_t total_vsize) const EXCLUSIVE_LOCKS_REQUIRED(cs);