[clang] [analyzer][NFC] Document getVarRegion shortcomings (PR #191168)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

It turns out that `ParamVarRegion` already had a doc-comment which explained that certain parameters (entrypoint frame + captured ones) are not represented by `ParamVarRegion`s.

I still think that it is useful to add these new TODO comments, because (as my example shows) there is no guarantee that the reader will notice the old one.

https://github.com/llvm/llvm-project/pull/191168