Auto merge of #13756 - dtolnay-contrib:testdoc, r=weihanglo

Recategorize cargo test's `--doc` flag under "Target Selection"

### What does this PR try to resolve?

In `cargo help test`, the `--doc` flag is listed under a section called "Target Selection" next to `--lib`, `--bin`, `--bins`, `--example`, `--examples`, `--test`, `--tests`, `--bench`, `--benches`, and `--all-targets`.

But in `cargo test --help`, it was instead listed in an "Options" section next to `--no-run`, `--message-format`, `--color`, etc, which seems less appropriate than "Target Selection".

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

- `cargo build --release`
- `cargo test --release --test testsuite -- cargo_test::help::case`
- `target/release/cargo test --help`
- `target/release/cargo help test` (unchanged)

src/bin/cargo/commands/test.rs

@@ -18,7 +18,6 @@ pub fn cli() -> Command {
                 .num_args(0..)
                 .last(true),
         )
-        .arg(flag("doc", "Test only this library's documentation"))
         .arg(flag("no-run", "Compile, but don't run tests"))
         .arg(flag("no-fail-fast", "Run all tests regardless of failure"))
         .arg_future_incompat_report()
@@ -47,6 +46,10 @@ pub fn cli() -> Command {
             "Test all bench targets",
             "Test all targets (does not include doctests)",
         )
+        .arg(
+            flag("doc", "Test only this library's documentation")
+                .help_heading(heading::TARGET_SELECTION),
+        )
         .arg_features()
         .arg_jobs()
         .arg_unsupported_keep_going()