Mention an extra argument required to check tests

Add a note to tests-related lint descriptions that clippy needs to be
run with `--tests` argument to actually check tests, which is not quite
obvious.

Author: AMDmi3

clippy_lints/src/attrs/mod.rs

@@ -453,6 +453,8 @@ declare_clippy_lint! {
     /// ### What it does
     /// Checks for ignored tests without messages.
     ///
+    /// Note: you need to run `cargo clippy --tests` to enable tests checking.
+    ///
     /// ### Why is this bad?
     /// The reason for ignoring the test may not be obvious.
     ///

clippy_lints/src/redundant_test_prefix.rs

@@ -17,6 +17,8 @@ declare_clippy_lint! {
     /// Checks for test functions (functions annotated with `#[test]`) that are prefixed
     /// with `test_` which is redundant.
     ///
+    /// Note: you need to run `cargo clippy --tests` to enable tests checking.
+    ///
     /// ### Why is this bad?
     /// This is redundant because test functions are already annotated with `#[test]`.
     /// Moreover, it clutters the output of `cargo test` since test functions are expanded as