[clang] [analyzer][NFC] Add some docs for LazyCompoundValue (PR #97407)

[Kristóf Umann via cfe-commits]

Szelethus wrote:

I'd also love to see some docs for `Loc` and `NonLoc`, because I recall some angry looks from years ago when I touched those on their own, but I can barely recall them :( As well as some docs for the various `ProgramState::getSVal` methods...

https://github.com/llvm/llvm-project/pull/97407