db: add comments to roLimiter field (#13921)

ust to not worry about them in tests and small cli tools

erigon-lib/kv/mdbx/kv_mdbx.go

@@ -63,7 +63,6 @@ type MdbxOpts struct {
 	// must be in the range from 12.5% (almost empty) to 50% (half empty)
 	// which corresponds to the range from 8192 and to 32768 in units respectively
 	log             log.Logger
-	roTxsLimiter    *semaphore.Weighted
 	bucketsCfg      TableCfgFunc
 	path            string
 	syncPeriod      time.Duration
@@ -78,6 +77,11 @@ type MdbxOpts struct {
 	label           kv.Label // marker to distinct db instances - one process may open many databases. for example to collect metrics of only 1 database
 	inMem           bool
 
+	// roTxsLimiter - without this limiter - it's possible to reach 10K threads (if 10K rotx will wait for IO) - and golang will crush https://groups.google.com/g/golang-dev/c/igMoDruWNwo
+	// most of db must set explicit `roTxsLimiter <= 9K`.
+	// There is way to increase the 10,000 thread limit: https://golang.org/pkg/runtime/debug/#SetMaxThreads
+	roTxsLimiter *semaphore.Weighted
+
 	metrics bool
 }