Auto merge of rust-lang#11574 - unexge:missing-assert-message-docs, r=Alexendoo

Mention that `missing_assert_message` lint ignores test functions

Updates `missing_assert_message`'s docs to reflect that it ignores test functions as pointed out by `@mickvangelderen` in rust-lang/rust-clippy#10362 (comment)

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changelog: [`missing_assert_message`]: Update docs to reflect this lint ignores test functions

Author: bors

clippy_lints/src/missing_assert_message.rs

@@ -15,6 +15,10 @@ declare_clippy_lint! {
     /// A good custom message should be more about why the failure of the assertion is problematic
     /// and not what is failed because the assertion already conveys that.
     ///
+    /// Although the same reasoning applies to testing functions, this lint ignores them as they would be too noisy.
+    /// Also, in most cases understanding the test failure would be easier
+    /// compared to understanding a complex invariant distributed around the codebase.
+    ///
     /// ### Known problems
     /// This lint cannot check the quality of the custom panic messages.
     /// Hence, you can suppress this lint simply by adding placeholder messages