docs: clarify env inclusion in bundles

[jimCresswell]

Clarify that all next.config.js envs are included in the client-side bundle.

I recently came across an issue where the instructions for specifying environment variables in the next.config.js file and the instructions for specifying them in .env files were conflated, and a value was included in the client-side bundle that was not supposed to be there.

Documentation / Examples

• [x] Make sure the linting passes by running pnpm lint
• [x] The "examples guidelines" are followed from our contributing doc

[jimCresswell]

cc @mcky

[balazsorban44]

FWIW, the recommendation is to use https://nextjs.org/docs/basic-features/environment-variables which has safety checks to exclude environment variables that are only meant for the server.

[jimCresswell]

FWIW, the recommendation is to use https://nextjs.org/docs/basic-features/environment-variables which has safety checks to exclude environment variables that are only meant for the server.

@balazsorban44 absolutely, I just wanted to clarify that the safeguarding mechanism doesn't work for the old approach.