clang-format: Add option to always break after a function's return type.

This is required for GNU coding style, among others.

Also update the configuration documentation.

Modified from an original patch by Jarkko Hietaniemi, thank you!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@214858 91177308-0d34-0410-b5e6-96231b3b80d8