docs: clarify environment variables behavior (#36620)

Based on #36551 (comment) making the docs more clear which also aligns with the wording of Environment Variables in Middlerware: https://nextjs.org/docs/api-reference/next/server#how-do-i-access-environment-variables

See also: #20869 (comment)

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes by running `yarn lint`


Co-authored-by: JJ Kasper <22380829+ijjk@users.noreply.github.com>

docs/basic-features/environment-variables.md

@@ -46,8 +46,7 @@ export async function getStaticProps() {
 }
 ```
 
-> **Note**: In order to keep server-only secrets safe, Next.js replaces `process.env.*` with the correct values
-> at build time. This means that `process.env` is not a standard JavaScript object, so you’re not able to
+> **Note**: In order to keep server-only secrets safe, environment variables are evaluated at build time, so only environment variables _actually_ used will be included. This means that `process.env` is not a standard JavaScript object, so you’re not able to
 > use [object destructuring](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment).
 > Environment variables must be referenced as e.g. `process.env.PUBLISHABLE_KEY`, _not_ `const { PUBLISHABLE_KEY } = process.env`.