[NFC] Fix some docs warnings

Summary:
Fixes two minor issues in the docs present under `ninja docs-llvm-html`:

1 - A header is too small:
```
Warning, treated as error:
llvm/llvm/docs/Passes.rst:70:Title underline too short.

``-basic-aa``: Basic Alias Analysis (stateless AA impl)
------------------------------------------------------
```

2 - Multiple definitions on a non-anonymous target (llvm-dev mailing list):
```
Warning, treated as error:
llvm/llvm/docs/DeveloperPolicy.rst:3:Duplicate explicit target name: "llvm-dev mailing list".
```

Reviewers: lattner

Reviewed By: lattner

Subscribers: llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D83416