Add test about missing docs at crate level

Author: Urgau

Diff for: tests/ui/lint/lint-missing-doc-crate.rs

@@ -0,0 +1,4 @@
+// This test checks that we lint on the crate when it's missing a documentation.
+//
+//@ compile-flags: -Dmissing-docs --crate-type=lib
+//~ ERROR missing documentation for the crate

Diff for: tests/ui/lint/lint-missing-doc-crate.stderr

@@ -0,0 +1,10 @@
+error: missing documentation for the crate
+  --> $DIR/lint-missing-doc-crate.rs:4:47
+   |
+LL |
+   |                                              ^
+   |
+   = note: requested on the command line with `-D missing-docs`
+
+error: aborting due to 1 previous error
+