Improve description for `flatJson` option

[NamesMT]

Clear and concise description of the problem

The current description for flatJson option is: "Allow use flat json messages or not", which is not correct.
It's more of "Allow use of nested/mixed flat and hierarchical json", and what the option actually does is recursively transforming any flat key into a hierachical key.

In the current state of vue-i18n, if the json messages is a simple one level deep flat json (a.k.a key:value json), it is not necessary to enable this option and it would save some overhead.

More context:
flatJson will modify the original object passed in and can cause some gotchas.
flatJson can cause confusion with petite-vue-i18n, and if enabled, will broke petite-vue-i18n when working with flat json.

Suggested solution

TBD

Alternative

No response

Additional context

No response

Validations

• Read the Contributing Guidelines
• Read the Documentation
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.