Update config docs (#5265)

Minor updates to `PARALLEL_LEDGER_APPLY` to reflect current
non-experimental status; also, removes the `(experimental)` note from
`BACKGROUND_OVERLAY_PROCESSING`'s comment

Author: marta-lokhova

docs/stellar-core_example.cfg

@@ -275,10 +275,11 @@ BUCKETLIST_DB_PERSIST_INDEX = true
 # thread.
 BACKGROUND_OVERLAY_PROCESSING = true
 
-# EXPERIMENTAL_PARALLEL_LEDGER_APPLY (bool) default false
+# PARALLEL_LEDGER_APPLY (bool) default true
 # This causes ledger application to be done in parallel, which can lead to better
-# performance on multicore machines. Note that this is not compatible with SQLite.
-EXPERIMENTAL_PARALLEL_LEDGER_APPLY = false
+# performance on multicore machines. Note that this is not compatible with
+# in-memory SQLite.
+PARALLEL_LEDGER_APPLY = true
 
 # EXPERIMENTAL_PARALLEL_TX_SET_DOWNLOAD (bool) default false
 # Allow downloading of transaction sets in parallel with SCP

src/main/Config.h

@@ -526,10 +526,10 @@ class Config : public std::enable_shared_from_this<Config>
     // index.
     size_t BUCKETLIST_DB_INDEX_CUTOFF;
 
-    // Enable parallel processing of overlay operations (experimental)
+    // Enable parallel processing of overlay operations
     bool BACKGROUND_OVERLAY_PROCESSING;
 
-    // Enable parallel block application (experimental)
+    // Enable parallel block application
     bool PARALLEL_LEDGER_APPLY;
 
     // Allow downloading of transaction sets in parallel with SCP (experimental)