[clang-format] update comments in clang-format.py for python3 compatibility

Summary:
D23319 introduced python3 compatibility to clang-format.py, this is however not reflected by the documentation in the comments at the beginning of the file, which show how to use the script with python2 in .vimrc. While the actual mapping a user might want to use may well differ from my suggestion, I think it's nice to show the python2 and python3 version, such that a user can pick from the suggestions instead of googeling the python3 replacement for `:pyf` which they might not be familiar with.

EDIT: picking reviewers according to https://llvm.org/docs/Phabricator.html#finding-potential-reviewers

Reviewers: klimek, MyDeveloperDay

Reviewed By: MyDeveloperDay

Subscribers: ilya-biryukov, cfe-commits, llvm-commits, ychen

Patch By: pseyfert

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

Differential Revision: https://reviews.llvm.org/D38446