[clang-tidy] fix building clang-tidy documentation.

Summary:
The clang-tidy documentation can't be generated because of broken links.

```
Warning, treated as error:
/home/etienneb/llvm/llvm/tools/clang/tools/extra/docs/clang-tidy/checks/google-readability-function-size.rst:: WARNING: document isn't included in any toctree
```

Reviewers: alexfh

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D18803

llvm-svn: 265539