Vocoder matching fails without -c text.split_text=false

### Bug description

We can't use chunking for generating the synthesized_spec used for vocoder matching, so the docs should really have `-c text.split_text=false` as a mandatory flag.

### How to reproduce the bug

```python
# Sample code to reproduce the problem
```

### Error messages and logs

```
# Error messages and logs here please
```

### Environment

Current environment

# Please paste the output of `everyvoice --diagnostic` here

# EveryVoice Diagnostic information

### More info

_No response_

Locate the documentation for vocoder matching and the entry point that generates the synthesized_spec, then run the documented workflow with and without `-c text.split_text=false`. Update the relevant docs to mark the flag as required and verify that vocoder matching works with the documented command.

Written by the indexing model from the issue text.