Added documentation for auto_gc_behavior

go/cmd/dolt/commands/sqlserver/server_test.go

@@ -566,6 +566,8 @@ func TestGenerateYamlConfig(t *testing.T) {
   # disable_client_multi_statements: false
   # dolt_transaction_commit: false
   # event_scheduler: "OFF"
+  # auto_gc_behavior:
+    # enable: false
 
 listener:
   # host: localhost

go/cmd/dolt/commands/sqlserver/sqlserver.go

@@ -97,6 +97,8 @@ SUPPORTED CONFIG FILE FIELDS:
 
 {{.EmphasisLeft}}behavior.dolt_transaction_commit{{.EmphasisRight}}: If true all SQL transaction commits will automatically create a Dolt commit, with a generated commit message. This is useful when a system working with Dolt wants to create versioned data, but doesn't want to directly use Dolt features such as dolt_commit(). 
 
+{{.EmphasisLeft}}behavior.auto_gc_behavior.enabled{{.EmphasisRight}}: If true, garbage collection will run automatically in the background. 
+
 {{.EmphasisLeft}}listener.host{{.EmphasisRight}}: The host address that the server will run on.  This may be {{.EmphasisLeft}}localhost{{.EmphasisRight}} or an IPv4 or IPv6 address
 
 {{.EmphasisLeft}}listener.port{{.EmphasisRight}}: The port that the server should listen on

go/libraries/doltcore/servercfg/yaml_config.go

@@ -474,6 +474,9 @@ func (cfg YAMLConfig) withDefaultsFilledIn() YAMLConfig {
 	if withDefaults.BehaviorConfig.DoltTransactionCommit == nil {
 		withDefaults.BehaviorConfig.DoltTransactionCommit = defaults.BehaviorConfig.DoltTransactionCommit
 	}
+	if withDefaults.BehaviorConfig.AutoGCBehavior == nil {
+		withDefaults.BehaviorConfig.AutoGCBehavior = defaults.BehaviorConfig.AutoGCBehavior
+	}
 
 	if withDefaults.ListenerConfig.HostStr == nil {
 		withDefaults.ListenerConfig.HostStr = defaults.ListenerConfig.HostStr