Doc: improve documentation on merging user data sections

# Bug report
I am following the doc [1] to merge two user data, but it doesn't work.

[1] https://cloudinit.readthedocs.io/en/latest/reference/merging.html#example-cloud-config

## Steps to reproduce the problem
Here are my steps to reproduce this problem.

1, create a lxd test container with my-user-data.yaml

```
cat << EOF |tee /tmp/my-user-data.yaml
#cloud-config
merge_how:
- name: list
settings: [append]
- name: dict
settings: [no_replace, recurse_list]
runcmd:
- echo 'running command with runcmd from userdata' >> /var/log/cloud-init.log
EOF
lxc launch ubuntu:jammy test --config=user.user-data="$(cat /tmp/my-user-data.yaml)"

root@test:~# grep -r 'running command' /var/log/cloud-init.log
running command with runcmd from userdata
```

2, create another user-data (/etc/cloud/cloud.cfg.d/91_my.cfg) inside lxd container
```
cat << EOF |tee /etc/cloud/cloud.cfg.d/91_my.cfg
#cloud-config
merge_how:
- name: list
settings: [append]
- name: dict
settings: [no_replace, recurse_list]
runcmd:
- echo 'running command with runcmd from userdata' >> /var/log/cloud-init.log
EOF
```

3, restaart lxd container inside lxd container
```
cloud-init clean --logs --reboot
```

4, relogin lxd container again with 'lxc shell test', confirm both /etc/cloud/cloud.cfg.d/91_my.cfg and /var/lib/cloud/instances/90a1a29a-1f70-471a-a920-d1a6919270d0/user-data.txt are there
```
root@test:~# cat /etc/cloud/cloud.cfg.d/91_my.cfg
#cloud-config
merge_how:
- name: list
settings: [append]
- name: dict
settings: [no_replace, recurse_list]
runcmd:
- echo 'running command with runcmd from userdata' >> /var/log/cloud-init.log
root@test:~# cat /var/lib/cloud/instances/90a1a29a-1f70-471a-a920-d1a6919270d0/user-data.txt
#cloud-config
merge_how:
- name: list
settings: [append]
- name: dict
settings: [no_replace, recurse_list]
runcmd:
- echo 'running command with runcmd from userdata' >> /var/log/cloud-init.log
```

5, but the file /var/lib/cloud/instances/90a1a29a-1f70-471a-a920-d1a6919270d0/user-data.txt just override the file /etc/cloud/cloud.cfg.d/91_my.cfg, they haven't merged.
```
root@test:~# grep -r 'running command' /var/log/cloud-init.log
running command with runcmd from userdata
```

6, continue to do some checks
```
root@test:~# cloud-init query userdata
#cloud-config
merge_how:
- name: list
settings: [append]
- name: dict
settings: [no_replace, recurse_list]
runcmd:
- echo 'running command with runcmd from userdata' >> /var/log/cloud-init.log

root@test:~# cloud-init schema --system --annotate
Found cloud-config data types: user-data, network-config

1. user-data at /var/lib/cloud/instances/90a1a29a-1f70-471a-a920-d1a6919270d0/cloud-config.txt:
Valid schema user-data

2. network-config at /var/lib/cloud/instances/90a1a29a-1f70-471a-a920-d1a6919270d0/network-config.json:
Valid schema network-config
```

## Environment details
- Cloud-init version: 23.4.4-0ubuntu0~22.04.1
- Operating System Distribution: ubuntu 22.04
- Cloud provider, platform or installer type: lxd

## cloud-init logs
[cloud-init.log](https://github.com/canonical/cloud-init/files/14786015/cloud-init.log)