Document that cargo automatically registers variables used in env! macro to trigger rebuilds

[Imberflur]

Initially discussed in https://rust-lang.zulipchat.com/#narrow/channel/246057-t-cargo/topic/env!.2Foption_env!.20are.20tracked.2C.20can.20documentation.20be.20improved.3F/near/491793318

What does this PR try to resolve?

There is no documentation that I can find which indicates that cargo will rebuild for changes in the variables used in env! macros.

Because this was not always the case, when searching for information on this, the main result indicates otherwise https://users.rust-lang.org/t/should-env-cause-rebuild-is-env-var-changes/18013.

Users misled by this may turn to rerun-if-changed-env in a build script to trigger rebuilds, so this documentation is a useful place to catch users and indicate that it isn't needed for env! macros.

How should we test and review this PR?

I'm not sure how to test the new relative link but I matched it to the existing [env-macro]: ../../std/macro.env.html.

My main concern when writing this was whether it was clear that this applies to env! macro usage within the main code of the crate and not just in build.rs.

[rustbot]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @epage (or someone else) some time within the next two weeks.

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[weihanglo]

Probably flaky. Return tests.

---- test::panic_abort_tests stdout ----
running `/home/runner/work/cargo/cargo/target/debug/cargo test -Z panic-abort-tests -v`

thread 'test::panic_abort_tests' panicked at tests/testsuite/test.rs:5023:10:

---- expected: tests/testsuite/test.rs:5014:13
++++ actual:   stderr
   1    1 | [RUNNING] `[..]--crate-name a [..]-C panic=abort[..]`
   2    2 | [RUNNING] `[..]--crate-name foo [..]-C panic=abort[..]`
   3      - [RUNNING] `[..]--crate-name foo [..]-C panic=abort[..]--test[..]`
   4    3 | ...∅

[Imberflur]

I forgot to check here first and pushed to try to rerun the tests.

[weihanglo]

I forgot to check here first and pushed to try to rerun the tests.

Nah. I added the comment just for a record, since I clicked the rerun button in GitHub CI dashboard.