Document changed behavior in 205a43dfb ("Make argument to tst_umount an
absolute path").

Signed-off-by: Petr Vorel <pvorel@suse.cz>
---
 doc/shell-test-api.txt | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/doc/shell-test-api.txt b/doc/shell-test-api.txt
index ecdbe9e6a..afc01dd01 100644
--- a/doc/shell-test-api.txt
+++ b/doc/shell-test-api.txt

@@ -515,8 +515,8 @@ The 'tst_mount' mounts '$TST_DEVICE' of '$TST_FS_TYPE' (optional) to
 '$TST_MNT_PARAMS'. The '$TST_MNTPOINT' directory is created if it didn't
 exist prior to the function call.
 
-If the path passed (optional, defaults to '$TST_MNTPOINT') to the 'tst_umount' is
-not mounted (present in '/proc/mounts') it's noop.
+If the path passed (optional, should be full path, defaults to '$TST_MNTPOINT')