document new series of internal_write measurement

### Relevant telegraf.conf

```toml
[agent]
hostname = "myserver"

[[inputs.internal]]
namepass = ["internal_write"]

[[outputs.file]]
files = [ "stdout" ]
```

### Logs from Telegraf

```text
internal_write,host=myserver,input=internal,version=1.32.3 startup_errors=0i 1738793420000000000
internal_write,host=myserver,output=file,version=1.32.3 write_time_ns=0i,startup_errors=0i,errors=0i,metrics_dropped=0i,metrics_added=0i,metrics_filtered=0i,metrics_written=0i,buffer_limit=10000i,buffer_size=0i 1738793420000000000
```

### System info

Telegraf v1.32.3

### Docker

_No response_

### Steps to reproduce

1. Just run telegraf, wait a little and watch output
`telegraf --config internal_write.conf`

### Expected behavior

According to current [README](https://github.com/influxdata/telegraf/blob/v1.32.3/plugins/inputs/internal/README.md) the output should be just:
`internal_write,host=myserver,output=file,version=1.32.3 write_time_ns=0i,startup_errors=0i,errors=0i,metrics_dropped=0i,metrics_added=0i,metrics_filtered=0i,metrics_written=0i,buffer_limit=10000i,buffer_size=0i 1738793420000000000`

### Actual behavior

This series is not documented in [README](https://github.com/influxdata/telegraf/blob/master/plugins/inputs/internal/README.md):
`internal_write,host=myserver,input=internal,version=1.32.3 startup_errors=0i 1738793420000000000`
It would be nice to document it just for the sake of completeness. Thanks.

### Additional info

_No response_

Open plugins/inputs/internal/README.md and compare its documented internal_write series with the issue's Telegraf output. Document the input=internal series with its startup_errors field, then run the provided internal_write configuration and confirm both series are represented accurately.

Written by the indexing model from the issue text.