Flag Parse Server options which can only be used in certain setups

New Issue Checklist

• I am not disclosing a vulnerability.
• I am not just asking a question.
• I have searched through existing issues.
• I can reproduce the issue with the latest version of Parse Server.

Issue Description

Certain Parse Server options can only be used (and only have an effect) when starting Parse Server via the CLI. These options lack this restriction in their documentation. For example:

• trustProxy: has no effect when mounting Parse Server on a custom express app; in that case expressApp.set('trust proxy', ...); needs to be used instead
• clusters: can only be used when launching Parse Server via the CLI; has no effect when instantiating a single Parse Server instance

Solution

• Add limitation to the docs for affected options.
• If technically possible log a warning when an option is used in a way where it is ineffective

Start by reviewing the documentation for trustProxy and clusters, along with the CLI and custom Express initialization paths described in the issue. Identify other options with the same setup limitation, document their valid contexts, and assess whether ineffective use can safely produce a warning; done means the affected options clearly state these restrictions.