[clang] [NFC][analyzer] Clarify that ExplodedGraph has only one root (PR #139903)

[Donát Nagy via cfe-commits]

NagyDonat wrote:

(By the way, the commit message `Use` intended to be ```Use `getRootLocationContext()` ``` but the shell evaluated the backticks.)

https://github.com/llvm/llvm-project/pull/139903