[docs] Fix some Sphinx warnings.

The docs now build cleanly. Yay!

The following warnings were fixed:

/home/sean/pg/llvm/llvm/docs/HowToReleaseLLVM.rst:364: WARNING: Enumerated list ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/InAlloca.rst:: WARNING: document isn't included in any toctree

/home/sean/pg/llvm/llvm/docs/CodingStandards.rst:85: WARNING: Title underline too short.

Supported C++11 Language and Library Features
-------------------------------------------
/home/sean/pg/llvm/llvm/docs/CodingStandards.rst:85: WARNING: Title underline too short.

Supported C++11 Language and Library Features
-------------------------------------------
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:185: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:565: WARNING: Explicit markup ends without a blank line; unexpected unindent.
/home/sean/pg/llvm/llvm/docs/GettingStarted.rst:567: WARNING: Block quote ends without a blank line; unexpected unindent.