Mention that cargo test --all-targets excludes doctests

Per my proposal here: #6669 (comment)

I tried to keep the edit minimalistic to match the surrounding style.

If the maintainers are amenable to it, I think it could also be useful to do one or more of:
- Offer concrete guidance on what to do to run actually-all tests (`--all-targets` then separately `--doc`).
- Link to the issue at: #6669
- Mention that `cargo test` without `--all-targets` runs doctests by default, which seems not immediately obvious.

I'd be happy to attempt to add any of the above that the maintainers feel would be a good fit here.

src/bin/cargo/commands/test.rs

@@ -34,7 +34,7 @@ pub fn cli() -> Command {
             "Test all tests",
             "Test only the specified bench target",
             "Test all benches",
-            "Test all targets",
+            "Test all targets (does not include doctests)",
         )
         .arg(flag("doc", "Test only this library's documentation"))
         .arg(flag("no-run", "Compile, but don't run tests"))

tests/testsuite/cargo_test/help/stdout.log

@@ -17,7 +17,7 @@ Options:
       --test [<NAME>]           Test only the specified test target
       --benches                 Test all benches
       --bench [<NAME>]          Test only the specified bench target
-      --all-targets             Test all targets
+      --all-targets             Test all targets (does not include doctests)
       --doc                     Test only this library's documentation
       --no-run                  Compile, but don't run tests
       --no-fail-fast            Run all tests regardless of failure