[clang-format] Update dump_format_style.py to indent nested fields

Summary:
This updates the format options documentation script to indent the
documentation of nested fields. The previous format caused some problems,
as when a bulleted list ends with a multiline comment. See the buildbot failure
http://lab.llvm.org:8011/builders/clang-sphinx-docs/builds/10020/steps/docs-clang-html/logs/stdio

Reviewers: djasper

Reviewed By: djasper

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D34552

llvm-svn: 306093