[Docs] Add a link that refers to C++ standard modules in Clang modules doc

[junaire]

Summary

Currently there're two pages that both talk about "Modules" in clang, but
they're different. The one that describes C++ standard modules explicitly
spells out the difference but the other one which targeting Clang modules
doesn't.

This patch adds a link that refers to the C++ standard modules
one in Clang modules doc, as you usually got the later page when
googling. I believe this will make newcomers less confused.

Signed-off-by: Jun Zhang <[e-mail]>

[ChuanqiXu]

Thanks. I wanted to do this. I suggest to do the change in later sections. Since the document talks more than the clang modules extension. It also describes some general module properties and semantics. It is a pretty good document and from my point of view, it should be helpful for people who desire C++20 Modules to read this (at least the sections in the frontend).

BTW, have you tested to generate the document?

clang/docs/Modules.rst

104

[junaire]

I bet now it rendered correctly.

[junaire]

[ChuanqiXu]

clang/docs/Modules.rst

107–108

minor suggestion.

clang/docs/Modules.rst

 
 * **Namespaces**: Unlike in some languages, modules do not imply any notion of namespaces. Thus, a struct declared in one module will still conflict with a struct of the same name declared in a different module, just as they would if declared in two different headers. This aspect is important for backward compatibility, because (for example) the mangled names of entities in software libraries must not change when introducing modules.
 
 * **Binary distribution of modules**: Headers (particularly C++ headers) expose the full complexity of the language. Maintaining a stable binary module format across architectures, compiler versions, and compiler vendors is technically infeasible.
 
 Using Modules
 =============
 To enable modules, pass the command-line flag ``-fmodules``. This will make any modules-enabled software libraries available as modules as well as introducing any modules-specific syntax. Additional `command-line parameters`_ are described in a separate section later.
 
+Standard C++ Modules
+--------------------
+.. note::
+  Modules are adopted into C++20 Standard. And its semantic and command line interface are very different from the Clang C++ modules. See `StandardCPlusPlusModules <StandardCPlusPlusModules.html>`_ for details.
+
 Objective-C Import declaration
 ------------------------------
 Objective-C provides syntax for importing a module via an *@import declaration*, which imports the named module:
 
 .. parsed-literal::
 
   @import std;