doc: cmake: prepend and explain "build/" where needed

test/functional/README.md

@@ -10,7 +10,8 @@ that file and modify to fit your needs.
 
 #### Coverage
 
-Running `test/functional/test_runner.py` with the `--coverage` argument tracks which RPCs are
+Assuming the build directory is `build`,
+running `build/test/functional/test_runner.py` with the `--coverage` argument tracks which RPCs are
 called by the tests and prints a report of uncovered RPCs in the summary. This
 can be used (along with the `--extended` argument) to find out which RPCs we
 don't have test cases for.