Add a test for #3575

Author: topecongiro

tests/source/normalize_multiline_doc_attribute.rs

@@ -5,3 +5,8 @@
 is split
 on multiple lines"]
 fn foo() {}
+
+#[doc = " B1"]
+#[doc = ""]
+#[doc = " A1"]
+fn bar() {}

tests/target/normalize_multiline_doc_attribute.rs

@@ -5,3 +5,8 @@
 ///is split
 ///on multiple lines
 fn foo() {}
+
+/// B1
+///
+/// A1
+fn bar() {}