Fix docs and add overlap test for negated per-file-ignores (#10863)

Refs #3172 

## Summary

Fix a typo in the docs example, and add a test for the case where a
negative pattern and a positive pattern overlap.

The behavior here is simple: patterns (positive or negative) are always
additive if they hit (i.e. match for a positive pattern, don't match for
a negated pattern). We never "un-ignore" previously-ignored rules based
on a pattern (positive or negative) failing to hit.

It's simple enough that I don't really see other cases we need to add
tests for (the tests we have cover all branches in the ignores_from_path
function that implements the core logic), but open to reviewer feedback.

I also didn't end up changing the docs to explain this more, because I
think they are accurate as written and don't wrongly imply any more
complex behavior. Open to reviewer feedback on this as well!

After some discussion, I think allowing negative patterns to un-ignore
rules is too confusing and easy to get wrong; if we need that, we should
add `per-file-selects` instead.

## Test Plan

Test/docs only change; tests pass, docs render and look right.

---------

Co-authored-by: Alex Waygood <Alex.Waygood@gmail.com>

Author: carljm

crates/ruff/tests/lint.rs

@@ -1248,3 +1248,39 @@ fn negated_per_file_ignores_absolute() -> Result<()> {
     });
     Ok(())
 }
+
+/// patterns are additive, can't use negative patterns to "un-ignore"
+#[test]
+fn negated_per_file_ignores_overlap() -> Result<()> {
+    let tempdir = TempDir::new()?;
+    let ruff_toml = tempdir.path().join("ruff.toml");
+    fs::write(
+        &ruff_toml,
+        r#"
+[lint.per-file-ignores]
+"*.py" = ["RUF"]
+"!foo.py" = ["RUF"]
+"#,
+    )?;
+    let foo_file = tempdir.path().join("foo.py");
+    fs::write(foo_file, "")?;
+    let bar_file = tempdir.path().join("bar.py");
+    fs::write(bar_file, "")?;
+
+    assert_cmd_snapshot!(Command::new(get_cargo_bin(BIN_NAME))
+        .args(STDIN_BASE_OPTIONS)
+        .arg("--config")
+        .arg(&ruff_toml)
+        .arg("--select")
+        .arg("RUF901")
+        .current_dir(&tempdir)
+        , @r###"
+    success: true
+    exit_code: 0
+    ----- stdout -----
+    All checks passed!
+
+    ----- stderr -----
+    "###);
+    Ok(())
+}

crates/ruff_workspace/src/options.rs

@@ -916,7 +916,7 @@ pub struct LintCommonOptions {
             "__init__.py" = ["E402"]
             "path/to/file.py" = ["E402"]
             # Ignore `D` rules everywhere except for the `src/` directory.
-            "!src/**.py" = ["F401"]
+            "!src/**.py" = ["D"]
         "#
     )]
     pub per_file_ignores: Option<FxHashMap<String, Vec<RuleSelector>>>,