Update ECS_RESERVED_MEMORY description in README (#3363)

Author: Heming Han

README.md

@@ -142,7 +142,7 @@ additional details on each available environment variable.
 | `ECS_POLL_METRICS`     | <true | false>  | Whether to poll or stream when gathering metrics for tasks. Setting this value to `true` can help reduce the CPU usage of dockerd and containerd on the ECS container instance. See also ECS_POLL_METRICS_WAIT_DURATION for setting the poll interval. | `false` | `false` |
 | `ECS_POLLING_METRICS_WAIT_DURATION` | 10s | Time to wait between polling for metrics for a task. Not used when ECS_POLL_METRICS is false. Maximum value is 20s and minimum value is 5s. If user sets above maximum it will be set to max, and if below minimum it will be set to min. | 10s | 10s |
 | `ECS_PULL_DEPENDENT_CONTAINERS_UPFRONT` | <true | false> | Whether to pull images for containers with dependencies before the dependsOn condition has been satisfied. | false | false |
-| `ECS_RESERVED_MEMORY` | 32 | Memory, in MiB, to reserve for use by things other than containers managed by Amazon ECS. | 0 | 0 |
+| `ECS_RESERVED_MEMORY` | 32 | Reduction, in MiB, of the memory capacity of the instance that is reported to Amazon ECS. Used by Amazon ECS when placing tasks on container instances. This doesn't reserve memory usage on the instance. | 0 | 0 |
 | `ECS_AVAILABLE_LOGGING_DRIVERS` | `["awslogs","fluentd","gelf","json-file","journald","logentries","splunk","syslog"]` | Which logging drivers are available on the container instance. | `["json-file","none"]` | `["json-file","none"]` |
 | `ECS_DISABLE_PRIVILEGED` | `true` | Whether launching privileged containers is disabled on the container instance. | `false` | `false` |
 | `ECS_SELINUX_CAPABLE` | `true` | Whether SELinux is available on the container instance. | `false` | `false` |

agent/config/types.go

@@ -101,8 +101,9 @@ type Config struct {
 	// on the instance
 	DisableDockerHealthCheck BooleanDefaultFalse
 
-	// ReservedMemory specifies the amount of memory (in MB) to reserve for things
-	// other than containers managed by ECS
+	// ReservedMemory specifies Reduction, in MiB, of the memory capacity of the instance
+	// that is reported to Amazon ECS. Used by Amazon ECS when placing tasks on container instances.
+	// This doesn't reserve memory usage on the instance
 	ReservedMemory uint16
 
 	// DockerStopTimeout specifies the amount of time before a SIGKILL is issued to