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

[Aaron Ballman via cfe-commits]

================
@@ -400,24 +389,27 @@ And the compilation process for module units are like:
                 mod1.cppm -> clang++ mod1.cppm ... -> mod1.pcm --,--> clang++ mod1.pcm ... -> mod1.o -+
                 src2.cpp ----------------------------------------+> clang++ src2.cpp -------> src2.o -'
 
-As the diagrams show, we need to compile the BMI from module units to object files and link the object files.
-(But we can't do this for the BMI from header units. See the later section for the definition of header units)
+As the diagrams show, we need to compile the BMI from module units to object
+files and then link the object files. (However, we can't do this for the BMI
+from header units. See the section on :ref:`header units <header-units>` for
+more details.
 
-If we want to create a module library, we can't just ship the BMIs in an archive.
-We must compile these BMIs(``*.pcm``) into object files(``*.o``) and add those object files to the archive instead.
+BMIs cannot be shipped in an archive to create a module library. Instead, the
+BMIs(``*.pcm``) are compiled into object files(``*.o``) and those object files
+are added to the archive instead.
 
-Consistency Requirement
-~~~~~~~~~~~~~~~~~~~~~~~
+Consistency Requirements
+~~~~~~~~~~~~~~~~~~~~~~~~
 
-If we envision modules as a cache to speed up compilation, then - as with other caching techniques -
-it is important to keep cache consistency.
-So **currently** Clang will do very strict check for consistency.
+If modules are thought of as a kind of cache to speed up compilation, then, as
----------------
AaronBallman wrote:

I went with:

Modules can be viewed as a kind of cache to speed up compilation. Thus, like
other caching techniques, it is important to maintain cache consistency which
is why Clang does very strict checking for consistency.


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