[clang-tidy] Add a useful note about -std=c++11-or-later

I and @whisperity spent some time debugging a LIT test case using the
`-std=c++11-or-later` `check_clang_tidy.py` flag when the test had
fixits.

It turns out if the test wants to report a diagnostic into a header
file AND into the test file as well, one needs to first copy the header
somewhere under the build directory.
It needs to be copied since `clang-tidy` sorts the reports into
alphabetical order, thus to have a deterministic order relative to the
diagnostic in the header AND the diagnostic in the test cpp file.

There is more to this story.

The `-std=c++11-or-later` turns out executes the test with multiple
`-std=XX` version substitution, and each execution will also have the
`-fix` tidy parameter. This means that the freshly copied header file I
stated in the previous paragraph gets fixed up and the very next tidy
execution will fail miserably.

Following @whisperity's advice, I'm leaving a reminder about such
//shared// state in the related doc comment.

Reviewed By: LegalizeAdulthood

Differential Revision: https://reviews.llvm.org/D125771