[clang] Revise the modules document for clarity (PR #90237)

[Erich Keane via cfe-commits]

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@@ -95,22 +76,23 @@ In this document, we use the following umbrella terms:
 * A ``module interface unit`` refers to either a ``primary module interface unit``
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erichkeane wrote:

^^ Line above but github wont let me: Don't use 'umbrella', its probably not a familiar term to foreign speakers.  Perhaps omit it entirely?

https://github.com/llvm/llvm-project/pull/90237