cxxmodules: rework control logic for scanning regular C++ sources

Now that scanning support is no longer experimental, the logic for
whether or not to scan C++ 20 sources is now important because all
projects are now exposted to the logic. Make the scanning rules explicit
in the documentation and rework the queries to localize all of the
associated logic.

A policy to handle the ultimate fallback logic will be implemented in a
following commit.

1 files changed, 17 insertions, 0 deletions

diff --git a/Help/manual/cmake-cxxmodules.7.rst b/Help/manual/cmake-cxxmodules.7.rst
index 91acae1..a082f58 100644
--- a/Help/manual/cmake-cxxmodules.7.rst
+++ b/Help/manual/cmake-cxxmodules.7.rst
@@ -12,6 +12,23 @@ to scan source files for module dependencies during the build, collates
 scanning results to infer ordering constraints, and tells the build tool
 how to dynamically update the build graph.
 
+Scanning Control
+================
+
+Whether or not sources get scanned for C++ module usage is dependent on the
+following queries. The first query that provides a yes/no answer is used.
+
+- If the source file belongs to a file set of type ``CXX_MODULES``, it will
+  be scanned.
+- If the target does not use at least C++ 20, it will not be scanned.
+- If the source file is not the language ``CXX``, it will not be scanned.
+- If the :prop_sf:`CXX_SCAN_FOR_MODULES` source file property is set, its
+  value will be used.
+- If the :prop_tgt:`CXX_SCAN_FOR_MODULES` target property is set, its value
+  will be used.  Set the :variable:`CMAKE_CXX_SCAN_FOR_MODULES` variable
+  to initialize this property on all targets as they are created.
+- Otherwise the source will be scanned.
+
 Compiler Support
 ================