Merge #13531: doc: Clarify that mempool txiter is const_iterator

faca0a8625 doc: Clarify that mempool txiter is const_iterator (MarcoFalke)

Pull request description:

  `iterator` and `const_iterator` are the same type for multi indexed transaction sets, but `const_iterator` should be preferred for documentation purposes.

Tree-SHA512: 83e8af36d15aa1e9fc59b3c2279504fd6f6ea3188dc43e36dec279ee0613ff07947d7143fd112bade7868b0dba59ecab3fd246cbde82e376ef965b646d9f8c4d

src/txmempool.h

@@ -486,7 +486,7 @@ public:
     mutable CCriticalSection cs;
     indexed_transaction_set mapTx GUARDED_BY(cs);
 
-    typedef indexed_transaction_set::nth_index<0>::type::iterator txiter;
+    using txiter = indexed_transaction_set::nth_index<0>::type::const_iterator;
     std::vector<std::pair<uint256, txiter> > vTxHashes; //!< All tx witness hashes/entries in mapTx, in random order
 
     struct CompareIteratorByHash {