add note about `cron.SCHEDULE` format in document (#15812)

* add note about ``cron.SCHEDULE`` format in document

Signed-off-by: a1012112796 <1012112796@qq.com>

* Update custom/conf/app.example.ini

Co-authored-by: zeripath <art27@cantab.net>

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

custom/conf/app.example.ini

@@ -1591,6 +1591,11 @@ PATH =
 ;ENABLED = false
 ;; Setting this to true will run all enabled cron tasks when Gitea starts.
 ;RUN_AT_START = false
+;;
+;; Note: ``SCHEDULE`` accept formats
+;;    - Full crontab specs, e.g. "* * * * * ?"
+;;    - Descriptors, e.g. "@midnight", "@every 1h30m"
+;; See more: https://pkg.go.dev/github.com/gogs/cron@v0.0.0-20171120032916-9f6c956d3e14
 
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Basic cron tasks - enabled by default

docs/content/doc/advanced/config-cheat-sheet.en-us.md

@@ -704,6 +704,11 @@ NB: You must have `DISABLE_ROUTER_LOG` set to `false` for this option to take ef
 - `RUN_AT_START`: **false**: Run cron tasks at application start-up.
 - `NO_SUCCESS_NOTICE`: **false**: Set to true to switch off success notices.
 
+- `SCHEDULE` accept formats
+   - Full crontab specs, e.g. `* * * * * ?`
+   - Descriptors, e.g. `@midnight`, `@every 1h30m` ...
+   - See more: [cron decument](https://pkg.go.dev/github.com/gogs/cron@v0.0.0-20171120032916-9f6c956d3e14)
+
 ### Basic cron tasks - enabled by default
 
 #### Cron - Cleanup old repository archives (`cron.archive_cleanup`)

docs/content/doc/advanced/config-cheat-sheet.zh-cn.md

@@ -245,6 +245,11 @@ test01.xls: application/vnd.ms-excel; charset=binary
 
 - `ENABLED`: 是否在后台运行定期任务。
 - `RUN_AT_START`: 是否启动时自动运行。
+- `SCHEDULE` 所接受的格式
+   - 完整 crontab 控制, 例如 `* * * * * ?`
+   - 描述符, 例如 `@midnight`, `@every 1h30m` ...
+   - 更多细节参见 [cron api文档](https://pkg.go.dev/github.com/gogs/cron@v0.0.0-20171120032916-9f6c956d3e14)
+
 
 ### Cron - Update Mirrors (`cron.update_mirrors`)