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

[Erich Keane via cfe-commits]

================
@@ -8,79 +8,60 @@ Standard C++ Modules
 Introduction
 ============
 
-The term ``modules`` has a lot of meanings. For the users of Clang, modules may
-refer to ``Objective-C Modules``, ``Clang C++ Modules`` (or ``Clang Header Modules``,
-etc.) or ``Standard C++ Modules``. The implementation of all these kinds of modules in Clang
-has a lot of shared code, but from the perspective of users, their semantics and
-command line interfaces are very different. This document focuses on
-an introduction of how to use standard C++ modules in Clang.
-
-There is already a detailed document about `Clang modules <Modules.html>`_, it
-should be helpful to read `Clang modules <Modules.html>`_ if you want to know
-more about the general idea of modules. Since standard C++ modules have different semantics
-(and work flows) from `Clang modules`, this page describes the background and use of
-Clang with standard C++ modules.
-
-Modules exist in two forms in the C++ Language Specification. They can refer to
-either "Named Modules" or to "Header Units". This document covers both forms.
+The term ``modules`` has a lot of meanings. For Clang users, modules may refer
+to ``Objective-C Modules``, `Clang Modules <Modules.html>`_ (also called
+``Clang Header Modules``, etc.) or ``C++20 Modules`` (or
+``Standard C++ Modules``). The implementation of all these kinds of modules in
+Clang shares a lot of code, but from the perspective of users, their semantics
+and command line interfaces are very different. This document focuses on an
----------------
erichkeane wrote:

```suggestion
and command line interfaces are very different. This document is an
```

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