Update document for configuarable percentiles

docs/cn/server.md

@@ -387,6 +387,21 @@ Server.set_version(...)可以为server设置一个名称+版本，可通过/vers
 
 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。
 
+## 定制延时的分位值
+
+显示的服务延时分位值**默认**为**80** (曾经为50), 90, 99, 99.9, 99.99，前三项可分别通过-bvar_latency_p1, -bvar_latency_p2, -bvar_latency_p3三个gflags定制。
+
+以下是正确的设置：
+```shell
+-bvar_latency_p3=97   # p3从默认99修改为97
+-bvar_latency_p1=60 -bvar_latency_p2=80 -bvar_latency_p3=95
+```
+以下是错误的设置：
+```shell
+-bvar_latency_p3=100   # 设置值必须在[1,99]闭区间内，gflags解析会失败
+-bvar_latency_p1=-1    # 同上
+```
+
 ## 限制最大消息
 
 为了保护server和client，当server收到的request或client收到的response过大时，server或client会拒收并关闭连接。此最大尺寸由[-max_body_size](http://brpc.baidu.com:8765/flags/max_body_size)控制，单位为字节。

docs/en/server.md

@@ -389,6 +389,20 @@ Set gflag -free_memory_to_system_interval to make the program try to return free
 
 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).
 
+## Customize percentiles of latency
+
+Latency percentiles showed are **80** (was 50 before), 90, 99, 99.9, 99.99 by default. The first 3 ones can be changed by gflags -bvar_latency_p1, -bvar_latency_p2, -bvar_latency_p3 respectively。
+
+Following are correct settings:
+```shell
+-bvar_latency_p3=97   # p3 is changed from default 99 to 97
+-bvar_latency_p1=60 -bvar_latency_p2=80 -bvar_latency_p3=95
+```
+Following are wrong settings:
+```shell
+-bvar_latency_p3=100   # the value must be inside [1,99] inclusive，otherwise gflags fails to parse
+-bvar_latency_p1=-1    # ^
+```
 ## Limit sizes of messages
 
 To protect servers and clients, when a request received by a server or a response received by a client is too large, the server or client rejects the message and closes the connection. The limit is controlled by [-max_body_size](http://brpc.baidu.com:8765/flags/max_body_size), in bytes.