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

[Aaron Ballman via cfe-commits]

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@@ -1230,6 +1230,57 @@ parsing their headers, those should be included after the import. If the
 imported modules don't provide such a header, one can be made manually for
 improved compile time performance.
 
+Reachability of internal partition units
+----------------------------------------
+
+The internal partition units are called as implementation partition unit somewhere else.
+But the name may be confusing since implementation partition units are not implementation
+units.
+
+According to [module.reach]p1,2:
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AaronBallman wrote:

It'd be good to specify which standard version you pulled the wording from.

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