Enhance the documentation for the --config-args options to all EV commands

### Description & Motivation

In `everyvoice/base_cli/interfaces.py`, and `everyvoice/base_cli/helpers.py` we have several base commands providing the `--config-args` option to override specific config values on the command line. This option is quite confusing to use and not sufficiently documented.

### Pitch

- [ ] Task 1 -- minimal improvement -- set `help="Override the configuration."` on all of them.

- [ ] Task 2 -- really help the user -- add an example in each case of a full valid way to use it.

### Additional context

Examples:

For train base command, we could use

help="Override the configuration, e.g., --config-args training.max_steps=200000 would set the number of training steps to 200k."

For inference shared parameters we could use

help="Override the configuration, e.g., --config-args text.split_text=False would disable text splitting."

And so on.

I'm purposefully choosing examples that:
1. I've used, so this would help me when I use them again, and also they might be cases that would realistically come up, and
2. are nested, to illustrate the syntax for setting deep parameters; if there was a parameter that was three levels deep that might be even better.