Elaborate on motivation of --unit-graph.

Author: ehuss

src/cargo/util/command_prelude.rs

@@ -451,7 +451,7 @@ pub trait ArgMatchesExt {
         if build_config.unit_graph {
             config
                 .cli_unstable()
-                .fail_if_stable_opt("--unit-graph", 0)?;
+                .fail_if_stable_opt("--unit-graph", 8002)?;
         }
 
         let opts = CompileOptions {

src/doc/src/reference/unstable.md

@@ -554,7 +554,7 @@ The `-Z crate-versions` flag will make `cargo doc` include appropriate crate ver
 You can find an example screenshot for the cargo itself in the tracking issue.
 
 ### unit-graph
-* Tracking Issue: TODO
+* Tracking Issue: [#8002](https://github.com/rust-lang/cargo/issues/8002)
 
 The `--unit-graph` flag can be passed to any build command (`build`, `check`,
 `run`, `test`, `bench`, `doc`, etc.) to emit a JSON object to stdout which
@@ -567,6 +567,15 @@ depends on.
 cargo +nightly build --unit-graph -Z unstable-options
 ```
 
+This structure provides a more complete view of the dependency relationship as
+Cargo sees it. In particular, the "features" field supports the new feature
+resolver where a dependency can be built multiple times with different
+features. `cargo metadata` fundamentally cannot represent the relationship of
+features between different dependency kinds, and features now depend on which
+command is run and which packages and targets are selected. Additionally it
+can provide details about intra-package dependencies like build scripts or
+tests.
+
 The following is a description of the JSON structure:
 
 ```javascript