[clang-format] update documentation

Summary:
  - Added example code for BreakStringLiterals;

Reviewers: MyDeveloperDay

Reviewed By: MyDeveloperDay

Patch By: mrexodia

Subscribers: cfe-commits, MyDeveloperDay

Tags: #clang-tools-extra, #clang-format, #clang

Differential Revision: https://reviews.llvm.org/D31574