reference: add TiDB cluster alert rules (#1792)

* releases: add 2.1.16 release notes

* Revert "releases: add 2.1.16 release notes"

This reverts commit 6a5d938.

* reference: add TiDB alert rules

* add the new file to TOC

* make descriptions more clear

* address comments

* change wording

* correct link format

* change wording to clarify meaning

* same changes in v3.0 and v2.1

* fix link format

* improve wording

* change punctuation marks

* left-align a table

* fix the table format

* change raft to Raft

* uppercase "node_exporter"

* fix more words

dev/TOC.md

@@ -287,6 +287,7 @@
     - [TiDB 面板](reference/key-monitoring-metrics/tidb-dashboard.md)
     - [PD 面板](reference/key-monitoring-metrics/pd-dashboard.md)
     - [TiKV 面板](reference/key-monitoring-metrics/tikv-dashboard.md)
+  - [报警规则](reference/alert-rules.md)
   + 安全
     - [与 MySQL 的安全特性差异](reference/security/compatibility.md)
     - [TiDB 数据库权限管理](reference/security/privilege-system.md)

dev/how-to/configure/memory-control.md

@@ -26,7 +26,7 @@ mem-quota-query = 34359738368
 此外还可通过如下几个 session 变量来控制一条 Query 中的内存使用，大多数用户只需要设置 `tidb_mem_quota_query` 即可，其他变量是高级配置，大多数用户不需要关心：
 
 | 变量名                            | 作用                                              | 单位  | 默认值    |
-|-----------------------------------|---------------------------------------------------|-------|-----------|
+|:-----------------------------------|:---------------------------------------------------|:-------|:-----------|
 | tidb_mem_quota_query              | 配置整条 SQL 的内存使用阈值                       | Byte  | 32 << 30  |
 | tidb_mem_quota_hashjoin           | 配置 Hash Join 的内存使用阈值                     | Byte  | 32 << 30  |
 | tidb_mem_quota_mergejoin          | 配置 Merge Join 的内存使用阈值                    | Byte  | 32 << 30  |