document that config/optional is safe to use with duplicate configuration

Problem/Motivation

The comments in ConfigInstaller::installOptionalConfig() say that optional config is ignored if it exists already:

      // Only list configuration that:
      // - does not already exist
      // - is a configuration entity (this also excludes config that has an
      //   implicit dependency on modules that are not yet installed)

However, this is only documented in this inline comment, and not in the 'config_api' docs topic in core.api.php.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

API changes

Data model changes

Release notes snippet