tests/doc: improvements to cleaning section

tests/README.md

@@ -63,12 +63,24 @@ Execution of multiple tests, especially all for particular board could generate
 many files. For example, after execution of all test for stm32f469i-disco board
 (more than 230 tests) around 7.5 GB of intermediate files are created.
 
-To delete all of this files go to main RIOT directory and execute command:
+There are few methods for cleaning intermediate files.
 
+If you would like to clean intermediate file only for particular board you should
+go to main RIOT directory and execute one from these commands:
 
     ./dist/tools/compile_and_test_for_board/compile_and_test_for_board.py . <board_of_your_choice> --compile-targets clean
 
-    make BOARD=<<board_of_your_choice> clean
+or
+
+    make BOARD=<board_of_your_choice> clean
+
+
+If you would like to clean intermediate files for all boards go to main RIOT 
+directory and use this command.
+
+@warning This command cleans all local files, for example, pkg downloads and
+locally generared docs.
+
 
     make distclean