[clang] [Doc] [C++20] [Modules] Clarify the reachability of internal partition units (PR #102572)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for the improved documentation! I made some suggestions for clarity, but please make sure I didn't change your intended meaning.

https://github.com/llvm/llvm-project/pull/102572