UBIFS: add a commentary about log recovery

Add a commentary which elaborates that 'ubifs_recover_log_leb()' recovers only
the last log LEB, not any. Also remove some unneeded newlines.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
diff --git a/fs/ubifs/recovery.c b/fs/ubifs/recovery.c
index c902a5d..77e9b87 100644
--- a/fs/ubifs/recovery.c
+++ b/fs/ubifs/recovery.c
@@ -772,7 +772,8 @@
  * @sbuf: LEB-sized buffer to use
  *
  * This function does a scan of a LEB, but caters for errors that might have
- * been caused by the unclean unmount from which we are attempting to recover.
+ * been caused by unclean reboots from which we are attempting to recover
+ * (assume that only the last log LEB can be corrupted by an unclean reboot).
  *
  * This function returns %0 on success and a negative error code on failure.
  */