Handle 'implementation safety' headers as well

clippy_lints/src/doc.rs

@@ -579,6 +579,8 @@ fn check_doc<'a, Events: Iterator<Item = (pulldown_cmark::Event<'a>, Range<usize
                     continue;
                 }
                 headers.safety |= in_heading && text.trim() == "Safety";
+                headers.safety |= in_heading && text.trim() == "Implementation safety";
+                headers.safety |= in_heading && text.trim() == "Implementation Safety";
                 headers.errors |= in_heading && text.trim() == "Errors";
                 headers.panics |= in_heading && text.trim() == "Panics";
                 if in_code {

tests/ui/doc_unsafe.rs

@@ -125,3 +125,8 @@ pub mod __macro {
         pub unsafe fn f() {}
     }
 }
+
+/// # Implementation safety
+pub unsafe trait DocumentedUnsafeTraitWithImplementationHeader {
+    fn method();
+}