Fix [env] relative description in reference (#15332)

<!--
Thanks for submitting a pull request 🎉! Here are some tips for you:

* If this is your first contribution, read "Cargo Contribution Guide"
first:
  https://doc.crates.io/contrib/
* Run `cargo fmt --all` to format your code changes.
* Small commits and pull requests are always preferable and easy to
review.
* If your idea is large and needs feedback from the community, read how:
  https://doc.crates.io/contrib/process/#working-on-large-features
* Cargo takes care of compatibility. Read our design principles:
  https://doc.crates.io/contrib/design.html
* When changing help text of cargo commands, follow the steps to
generate docs:

https://github.com/rust-lang/cargo/tree/master/src/doc#building-the-man-pages
* If your PR is not finished, set it as "draft" PR or add "WIP" in its
title.
* It's ok to use the CI resources to test your PR, but please don't
abuse them.

### What does this PR try to resolve?

Explain the motivation behind this change.
A clear overview along with an in-depth explanation are helpful.

You can use `Fixes #<issue number>` to associate this PR to an existing
issue.

### How should we test and review this PR?

Demonstrate how you test this change and guide reviewers through your
PR.
With a smooth review process, a pull request usually gets reviewed
quicker.

If you don't know how to write and run your tests, please read the
guide:
https://doc.crates.io/contrib/tests

### Additional information

Other information you want to mention in this PR, such as prior arts,
future extensions, an unresolved problem, or a TODO list.
-->

Current short description does not match reality nor the longer
description:
> Setting the relative flag evaluates the value as a config-relative
path that is relative to the parent directory of the .cargo directory
that contains the config.toml file. The value of the environment
variable will be the full absolute path.

Author: weihanglo

src/doc/src/reference/config.md

@@ -89,7 +89,7 @@ browser = "chromium"          # browser to use with `cargo doc --open`,
 ENV_VAR_NAME = "value"
 # Set even if already present in environment
 ENV_VAR_NAME_2 = { value = "value", force = true }
-# Value is relative to .cargo directory containing `config.toml`, make absolute
+# `value` is relative to the parent of `.cargo/config.toml`, env var will be the full absolute path
 ENV_VAR_NAME_3 = { value = "relative/path", relative = true }
 
 [future-incompat-report]