Document environment variables in cargo help

[devurandom]

According to the Cargo docs on crates.io, cargo build will be influenced by e.g. CARGO_TARGET_DIR, but cargo help does not document this.

Furthermore, the Cargo guide does not link to the documentation of the environment variables, which makes this feature rather hard to find.

[dwijnand]

Well the later half is no longer true. The Cargo guide now presents a sidebar TOC where the documentation for the environment variable is reachable.

[ehuss]

The documentation has been updated along the way to include more hints about environment variables. The --target flag at https://doc.rust-lang.org/cargo/commands/cargo-build.html mentions it. The cargo-build man page also has an Environment section which links to the reference. Embedding more detailed help in the built-in help output is tracked in #6104, so I'm going to close in favor of that for future improvements.