Reference-manual: compiler checks

Document that compiler checks are self-contained and
do not add arguments from anywhere else.

docs/markdown/Reference-manual.md

@@ -1636,6 +1636,12 @@ The following keyword arguments can be used:
   `sizeof`, `has_type`, `has_function`, `has_member`, `has_members`,
   `has_header_symbol`.
 
+**Note:** These compiler checks do not use compiler arguments added with 
+`add_*_arguments()`, via `-Dlang_args` on the command-line, or through
+`CFLAGS`/`LDFLAGS`, etc in the environment. Hence, you can trust that
+the tests will be fully self-contained, and won't fail because of custom
+flags added by other parts of the build file or by users.
+
 Note that if you have a single prefix with all your dependencies, you
 might find it easier to append to the environment variables
 `C_INCLUDE_PATH` with GCC/Clang and `INCLUDE` with MSVC to expand the