docs: document default_options behaviour

As discussed in issue #8037, using `c_args` in `project()` leads to
`CFLAGS` not being respected, which is a common mistake. Document this
and suggest using `add_project_arguments()` instead.

Signed-off-by: John Levon <levon@movementarian.org>

docs/yaml/functions/project.yaml

@@ -49,6 +49,11 @@ kwargs:
       the master project, settings in subprojects are ignored. Project
       specific options are used normally even in subprojects.
 
+      Note that some options can override the default behavior;
+      for example, using `c_args` here means that the `CFLAGS`
+      environment variable is not used. Consider using
+      [[add_project_arguments()]] instead.
+
   version:
     type: str | file
     description: |