Port docs on -free_memory_to_system_interval from wiki

docs/cn/server.md

@@ -379,6 +379,10 @@ Server.set_version(...)可以为server设置一个名称+版本，可通过/vers
 
 未打印日志的开销只是一次if判断，也不会评估参数(比如某个参数调用了函数，日志不打，这个函数就不会被调用）。如果日志最终打印到自定义LogSink，那么还要经过LogSink的过滤。
 
+## 归还空闲内存至系统
+
+选项-free_memory_to_system_interval表示每过这么多秒就尝试向系统归还空闲内存，<= 0表示不开启，默认值为0，若开启建议设为10及以上的值。此功能支持tcmalloc，之前程序中对`MallocExtension::instance()->ReleaseFreeMemory()`的定期调用可改成设置此选项。
+
 ## 打印发送给client的错误
 
 server的框架部分一般不针对个别client打印错误日志，因为当大量client出现错误时，可能导致server高频打印日志而严重影响性能。但有时为了调试问题，或就是需要让server打印错误，打开参数[-log_error_text](http://brpc.baidu.com:8765/flags/log_error_text)即可。

docs/en/server.md

@@ -381,6 +381,10 @@ Only logs with levels **not less than** the level specified by -minloglevel are
 
 Overhead of unprinted logs is just a "if" test and parameters are not evaluated (For example a parameter calls a function, if the log is not printed, the function is not called). Logs printed to LogSink may be filtered by the sink as well.
 
+## Return free memory to system
+
+Set gflag -free_memory_to_system_interval to make the program try to return free memory to system every so many seconds, values <= 0 disable the feature. Default value is 0. To turn it on, values >= 10 are recommended. This feature supports tcmalloc, thus `MallocExtension::instance()->ReleaseFreeMemory()` periodically called in your program can be replaced by setting this flag.
+
 ## Log error to clients
 
 Framework does not print logs for specific client generally, because a lot of errors caused by clients may slow down server significantly due to frequent printing of logs. If you need to debug or just want the server to log all errors, turn on [-log_error_text](http://brpc.baidu.com:8765/flags/log_error_text).