doc: add coinstatsindex to bitcoin.conf

Author: jonatack

share/examples/bitcoin.conf

@@ -142,8 +142,11 @@
 # both prior transactions and several dozen future transactions.
 #keypool=100
 
+# Maintain coinstats index used by the gettxoutsetinfo RPC (default: 0).
+#coinstatsindex=1
+
 # Enable pruning to reduce storage requirements by deleting old blocks.
-# This mode is incompatible with -txindex and -rescan.
+# This mode is incompatible with -txindex, -coinstatsindex and -rescan.
 # 0 = default (no pruning).
 # 1 = allows manual pruning via RPC.
 # >=550 = target to stay under in MiB.