[clang-tools-extra] [doc] Add documentation for clang-change-namespace (PR #148277)

[Konrad Kleine via cfe-commits]

kwk wrote:

@AaronBallman @tJener I've addressed all of you comments and hope this PR is now good to merge. Could you give it one last review? Am I supposed to mark the discussions we had as Resolved or is that up to you?

https://github.com/llvm/llvm-project/pull/148277