doc: Update test-writing-guidelines.txt

Add mention about newly introducet functions in shell library.

Signed-off-by: Cyril Hrubis <chrubis@suse.cz>
diff --git a/doc/test-writing-guidelines.txt b/doc/test-writing-guidelines.txt
index 2c40e3d..edd5905 100644
--- a/doc/test-writing-guidelines.txt
+++ b/doc/test-writing-guidelines.txt
@@ -1195,6 +1195,9 @@
 * tst_tmpdir()
 * tst_rmdir()
 * tst_fs_has_free()
+* tst_mkfs()
+* tst_acquire_device()
+* tst_release_device()
 
 There is one more function called 'tst_check_cmds()' that gets unspecified
 number of parameters and asserts that each parameter is a name of an