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

[Vlad Serebrennikov via cfe-commits]

================
@@ -633,36 +631,36 @@ example:
                                   // module M's interface, so is discarded
   int c = use_h<int>();           // OK
 
-In the above example, the function definition of ``N::g`` is elided from the Reduced
-BMI of ``M.cppm``. Then the use of ``use_g<int>`` in ``M-impl.cpp`` fails
-to instantiate. For such issues, users can add references to ``N::g`` in the module purview
-of ``M.cppm`` to make sure it is reachable, e.g., ``using N::g;``.
-
-We think the Reduced BMI is the correct direction. But given it is a drastic change,
-we'd like to make it experimental first to avoid breaking existing users. The roadmap
-of Reduced BMI may be:
-
-1. ``-fexperimental-modules-reduced-bmi`` is opt in for 1~2 releases. The period depends
-on testing feedbacks.
-2. We would announce Reduced BMI is not experimental and introduce ``-fmodules-reduced-bmi``.
-and suggest users to enable this mode. This may takes 1~2 releases too.
-3. Finally we will enable this by default. When that time comes, the term BMI will refer to
-the reduced BMI today and the Full BMI will only be meaningful to build systems which
-loves to support two phase compilations.
+In the above example, the function definition of ``N::g`` is elided from the
+Reduced BMI of ``M.cppm``. Then the use of ``use_g<int>`` in ``M-impl.cpp``
+fails to instantiate. For such issues, users can add references to ``N::g`` in
+the module purview of ``M.cppm`` to ensure it is reachable, e.g.,
+``using N::g;``.
+
+Long-term, Clang is likely to make Reduced BMIs the default rather than Full
+BMIs. But because it is a drastic change, it is initially an experimental
+option to avoid breaking existing users. The expected roadmap for Reduced BMIs
+as of Clang 19.x is:
+
+1. ``-fexperimental-modules-reduced-bmi`` is opt-in for 1~2 releases. The period depends
+   on user feedback and may be extended.
+2. Announce that Reduced BMIs are no longer experimental and introduce
+   ``-fmodules-reduced-bmi`` as a new option, and recommend use of the new
+   option. This transition is expected to take 1~2 additional releases as well.
+3. Finally, ``-fmodules-reduced-bmi`` will be the default. When that time
+   comes, the term BMI will refer to the reduced BMI and the Full BMI will only
----------------
Endilll wrote:

```suggestion
   comes, the term BMI will refer to the Reduced BMI and the Full BMI will only
```

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