Improve vitastor-nbd documentation

docs/usage/nbd.en.md

@@ -25,6 +25,23 @@ It will output a block device name like /dev/nbd0 which you can then use as a no
 
 You can also use `--pool <POOL> --inode <INODE> --size <SIZE>` instead of `--image <IMAGE>` if you want.
 
+Additional options for map command:
+
+* `--nbd_timeout 30` \
+  Timeout for I/O operations in seconds after exceeding which the kernel stops
+  the device. You can set it to 0 to disable the timeout, but beware that you
+  won't be able to stop the device at all if vitastor-nbd process dies.
+* `--nbd_max_devices 64 --nbd_max_part 3` \
+  Options for the `nbd` kernel module when modprobing it (`nbds_max` and `max_part`).
+  note that maximum allowed (nbds_max)*(1+max_part) is 256.
+* `--logfile /path/to/log/file.txt` \
+  Write log messages to the specified file instead of dropping them (in background mode)
+  or printing them to the standard output (in foreground mode).
+* `--dev_num N` \
+  Use the specified device /dev/nbdN instead of automatic selection.
+* `--foreground 1` \
+  Stay in foreground, do not daemonize.
+
 ## Unmap image
 
 To unmap the device run:
@@ -32,3 +49,27 @@ To unmap the device run:
 ```
 vitastor-nbd unmap /dev/nbd0
 ```
+
+## List mapped images
+
+```
+vitastor-nbd ls [--json]
+```
+
+Example output (normal format):
+
+```
+/dev/nbd0
+image: bench
+pid: 584536
+
+/dev/nbd1
+image: bench1
+pid: 584546
+```
+
+Example output (JSON format):
+
+```
+{"/dev/nbd0": {"image": "bench", "pid": 584536}, "/dev/nbd1": {"image": "bench1", "pid": 584546}}
+```