Move rustdoc UI tests into a subdirectory

This also adds a little leeway to the test limit.

src/test/ui/rustdoc/README.md

@@ -0,0 +1,3 @@
+This directory is for tests that have to do with rustdoc, but test the behavior
+of rustc. For example, rustc should not warn that an attribute rustdoc uses is
+unknown.

src/tools/tidy/src/ui_tests.rs

@@ -7,7 +7,7 @@
 
 const ENTRY_LIMIT: usize = 1000;
 // FIXME: The following limits should be reduced eventually.
-const ROOT_ENTRY_LIMIT: usize = 1418;
+const ROOT_ENTRY_LIMIT: usize = 1408;
 const ISSUES_ENTRY_LIMIT: usize = 2565;
 
 fn check_entries(path: &Path, bad: &mut bool) {