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

[Vlad Serebrennikov 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``
   or a ``module interface partition unit``.
 
-* An ``importable module unit`` refers to either a ``module interface unit``
-  or a ``internal module partition unit``.
+* An ``importable module unit`` refers to either a ``module interface unit`` or
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Endilll wrote:

This whole piece of documentation relies heavily on code font, and I feel we might be overusing it, like here. Maybe we should follow the example of the Standard, and use italics instead.

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