"conan lock create" does not append to lock file

### Environment details

OS: WSL Ubuntu 20.04
Conan: 2.0.0

### Steps to reproduce

Running:
`conan lock create --profile:build: xxx --rpfile:host yyy conanfile.py --lockfile-out new.lock`

According to https://docs.conan.io/2/reference/commands/lock/create.html:


I would expect my lock file to be extended every time I run lock create with different configs with that file as output.

What have I misunderstood?
How do I get it to extend the file?

Rgds
Klaus

### Logs

_No response_

Start with the lock create reference page linked in the report and compare its documented output behavior with the reproduced command. Clarify whether repeated runs append or overwrite, explain the supported way to obtain the intended result, and update the relevant documentation.

Written by the indexing model from the issue text.