doc: Correct docstring describing max block tree db cache

It is always applied in the same way, no matter how the txindex is
setup. This was no longer accurate after 8181db8, where their
initialization was made independent.

Author: TheCharlatan

src/txdb.h

@@ -27,11 +27,11 @@ static const int64_t nDefaultDbCache = 450;
 static const int64_t nDefaultDbBatchSize = 16 << 20;
 //! min. -dbcache (MiB)
 static const int64_t nMinDbCache = 4;
-//! Max memory allocated to block tree DB specific cache, if no -txindex (MiB)
+//! Max memory allocated to block tree DB specific cache (MiB)
 static const int64_t nMaxBlockDBCache = 2;
-//! Max memory allocated to block tree DB specific cache, if -txindex (MiB)
 // Unlike for the UTXO database, for the txindex scenario the leveldb cache make
 // a meaningful difference: https://github.com/bitcoin/bitcoin/pull/8273#issuecomment-229601991
+//! Max memory allocated to tx index DB specific cache in MiB.
 static const int64_t nMaxTxIndexCache = 1024;
 //! Max memory allocated to all block filter index caches combined in MiB.
 static const int64_t max_filter_index_cache = 1024;