[clang] [Doc] Update documentation for no-transitive-change (PR #96453)

[Aaron Ballman via cfe-commits]

================
@@ -652,6 +652,134 @@ in the future. The expected roadmap for Reduced BMIs as of Clang 19.x is:
    comes, the term BMI will refer to the Reduced BMI and the Full BMI will only
    be meaningful to build systems which elect to support two-phase compilation.
 
+Experimental No Transitive Change
+---------------------------------
+
+Starting with clang19.x, we introduced an experimental feature: the non-transitive
+change for modules, aimed at reducing unnecessary recompilations. For example,
+
+.. code-block:: c++
+
+  // m-partA.cppm
+  export module m:partA;
+
+  // m-partB.cppm
+  export module m:partB;
+  export int getB() { return 44; }
+
+  // m.cppm
+  export module m;
+  export import :partA;
+  export import :partB;
+
+  // useBOnly.cppm
+  export module useBOnly;
+  import m;
+  export int B() {
+    return getB();
+  }
+
+  // Use.cc
+  import useBOnly;
+  int get() {
+    return B();
+  }
+
+Now let's compile the project (For brevity, some commands are omitted.):
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 m-partA.cppm --precompile -o m-partA.pcm
+  $ clang++ -std=c++20 m-partB.cppm --precompile -o m-partB.pcm
+  $ clang++ -std=c++20 m.cppm --precompile -o m.pcm -fprebuilt-module-path=.
+  $ clang++ -std=c++20 useBOnly.cppm --precompile -o useBOnly.pcm -fprebuilt-module-path=.
+  $ md5sum useBOnly.pcm
+  07656bf4a6908626795729295f9608da  useBOnly.pcm
+
+then let's change the interface of ``m-partA.cppm`` to:
+
+.. code-block:: c++
+
+  // m-partA.v1.cppm
+  export module m:partA;
+  export int getA() { return 43; }
+
+Let's compile the BMI for `useBOnly` again:
+
+.. code-block:: console
+
+  $ clang++ -std=c++20 m-partA.cppm --precompile -o m-partA.pcm
+  $ clang++ -std=c++20 m-partB.cppm --precompile -o m-partB.pcm
+  $ clang++ -std=c++20 m.cppm --precompile -o m.pcm -fprebuilt-module-path=.
+  $ clang++ -std=c++20 useBOnly.cppm --precompile -o useBOnly.pcm -fprebuilt-module-path=.
+  $ md5sum useBOnly.pcm
+  07656bf4a6908626795729295f9608da  useBOnly.pcm
+
+We observed that the contents of useBOnly.pcm remain unchanged.
+Consequently, if the build system bases recompilation decisions on directly imported modules only,
+it becomes possible to skip the recompilation of Use.cc.
+It should be fine because the altered interfaces do not affect Use.cc in any way.
+This concept is called as no transitive changes.
----------------
AaronBallman wrote:

```suggestion
it becomes possible to skip the recompilation of ``Use.cc``
because the altered interfaces do not affect ``Use.cc`` in any way;
there are no transitive changes.
```

https://github.com/llvm/llvm-project/pull/96453