[clang-tools-extra] [NFC][clang-tidy] Extend doc-comment of BranchCloneCheck (PR #206116)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

This PR is a trivial update of a source code comment, so I'm merging it directly without wasting the attention of any reviewers.

https://github.com/llvm/llvm-project/pull/206116