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

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on the documentation for this, it's really appreciated! I had a few questions, but in general this is looking good.

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