Character restrictions aren't listed out for vault creation

### Where is the problem?

https://docs.konghq.com/gateway/latest/kong-enterprise/secrets-management/backends/env/

### What happened?

There are no character restrictions listed when trying to create a vault.
For example - When you try to create a vault that contains a "_" in the prefix, this is restricted and won't be allowed to create.

schema violation (prefix: invalid value: testing_vaults)

### What did you expect to happen?

Can we add the schema requirements to the docs page to avoid complications when creating a vault entity.

### Code of Conduct and Community Expectations

- [x] I agree to follow this project's Code of Conduct
- [x] I agree to abide by the Community Expectations