docs: read-only implies no-bgjob (#5562)

docs/en/reference/_common_options.mdx

@@ -6,7 +6,7 @@
 |`--backup-meta=3600`|interval (in seconds) to automatically backup metadata in the object storage (0 means disable backup) (default: "3600")|
 |`--backup-skip-trash` <VersionAdd>1.2</VersionAdd>|skip files and directories in trash when backup metadata.|
 |`--heartbeat=12`|interval (in seconds) to send heartbeat; it's recommended that all clients use the same heartbeat value (default: "12")|
-|`--read-only`|allow lookup/read operations only (default: false)|
+|`--read-only`|Read-only mode, i.e. allow only lookup/read operations. Note that this option implies `--no-bgjob`, so read-only clients do not execute background jobs.|
 |`--no-bgjob`|Disable background jobs, default to false, which means clients by default carry out background jobs, including:<br/><ul><li>Clean up expired files in Trash (look for `cleanupDeletedFiles`, `cleanupTrash` in [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go))</li><li>Delete slices that's not referenced (look for `cleanupSlices` in [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go))</li><li>Clean up stale client sessions (look for `CleanStaleSessions` in [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go))</li></ul>Note that compaction isn't affected by this option, it happens automatically with file reads and writes, client will check if compaction is in need, and run in background (take Redis for example, look for `compactChunk` in [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/redis.go)).|
 |`--atime-mode=noatime` <VersionAdd>1.1</VersionAdd> |Control atime (last time the file was accessed) behavior, support the following modes:<br/><ul><li>`noatime` (default): set when the file is created or when `SetAttr` is explicitly called. Accessing and modifying the file will not affect atime, tracking atime comes at a performance cost, so this is the default behavior</li><li>`relatime`: update inode access times relative to mtime (last time when the file data was modified) or ctime (last time when file metadata was changed). Only update atime if atime was earlier than the current mtime or ctime, or the file's atime is more than 1 day old</li><li>`strictatime`: always update atime on access</li></ul>|
 |`--skip-dir-nlink=20` <VersionAdd>1.1</VersionAdd> |number of retries after which the update of directory nlink will be skipped (used for tkv only, 0 means never) (default: 20)|

docs/zh_cn/reference/_common_options.mdx

@@ -6,7 +6,7 @@
 |`--backup-meta=3600`|自动备份元数据到对象存储的间隔时间；单位秒，默认 3600，设为 0 表示不备份。|
 |`--backup-skip-trash` <VersionAdd>1.2</VersionAdd>|备份元数据时跳过回收站中的文件和目录。|
 |`--heartbeat=12`|发送心跳的间隔（单位秒），建议所有客户端使用相同的心跳值 (默认：12)|
-|`--read-only`|启用只读模式挂载。|
+|`--read-only`|只读模式，只允许 lookup 和 read 请求。注意，只读模式隐含了 `--no-bgjob`，因此只读客户端不会运行后台任务。|
 |`--no-bgjob`|禁用后台任务，默认为 false，也就是说客户端会默认运行后台任务。后台任务包含：<br/><ul><li>清理回收站中过期的文件（在 [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go) 中搜索 `cleanupDeletedFiles` 和 `cleanupTrash`）</li><li>清理引用计数为 0 的 Slice（在 [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go) 中搜索 `cleanupSlices`）</li><li>清理过期的客户端会话（在 [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/base.go) 中搜索 `CleanStaleSessions`）</li></ul>特别地，与[企业版](https://juicefs.com/docs/zh/cloud/guide/background-job)不同，社区版碎片合并（Compaction）不受该选项的影响，而是随着文件读写操作，自动判断是否需要合并，然后异步执行（以 Redis 为例，在 [`pkg/meta/base.go`](https://github.com/juicedata/juicefs/blob/main/pkg/meta/redis.go) 中搜索 `compactChunk`）|
 |`--atime-mode=noatime` <VersionAdd>1.1</VersionAdd>|控制如何更新 atime（文件最后被访问的时间）。支持以下模式：<br/><ul><li>`noatime`（默认）：仅在文件创建和主动调用 `SetAttr` 时设置，平时访问与修改文件不影响 atime 值。考虑到更新 atime 需要运行额外的事务，对性能有影响，因此默认关闭。</li><li>`relatime`：仅在 mtime（文件内容修改时间）或 ctime（文件元数据修改时间）比 atime 新，或者 atime 超过 24 小时没有更新时进行更新。</li><li>`strictatime`：持续更新 atime</li></ul>|
 |`--skip-dir-nlink=20` <VersionAdd>1.1</VersionAdd>|跳过更新目录 nlink 前的重试次数 (仅用于 TKV, 0 代表永不跳过) (默认：20)|