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

[Chuanqi Xu 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
+introduction of focusing on the use of C++20 modules in Clang. In the remainder
+of this document, the term ``modules`` will refer to Standard C++20 modules and
+the term ``Clang modules`` will refer to the Clang modules extension.
+
+Modules exist in two forms in the C++ Standard. They can refer to either
+"Named Modules" or "Header Units". This document covers both forms.
 
 Standard C++ Named modules
 ==========================
 
-This document was intended to be a manual first and foremost, however, we consider it helpful to
-introduce some language background here for readers who are not familiar with
-the new language feature. This document is not intended to be a language
-tutorial; it will only introduce necessary concepts about the
-structure and building of the project.
+In order to understand compiler behavior, it is helpful to introduce some
+language background here for readers who are not familiar with the C++ feature.
+This document is not a tutorial on C++; it only introduces necessary concepts
+to better understand use of modules for a project.
 
 Background and terminology
 --------------------------
 
-Modules
-~~~~~~~
-
-In this document, the term ``Modules``/``modules`` refers to standard C++ modules
-feature if it is not decorated by ``Clang``.
-
-Clang Modules
-~~~~~~~~~~~~~
-
-In this document, the term ``Clang Modules``/``Clang modules`` refer to Clang
-c++ modules extension. These are also known as ``Clang header modules``,
-``Clang module map modules`` or ``Clang c++ modules``.
-
 Module and module unit
 ~~~~~~~~~~~~~~~~~~~~~~
 
-A module consists of one or more module units. A module unit is a special
-translation unit. Every module unit must have a module declaration. The syntax
-of the module declaration is:
+A module consists of one or more module units. A module unit is a special kind
+of translation unit. Every module unit must have a module declaration. The
----------------
ChuanqiXu9 wrote:

Technically not true: https://eel.is/c++draft/gram.basic#:translation-unit.

More specifically, a valid module unit may be:

```
module;
#include <whatever>
export module M;
...
```

Here the first `module;` keywords is not considered as module declaration.

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