Add explanation to restic diff symbols

Explain what the characters for each file in a restic diff output mean.

doc/040_backup.rst

@@ -498,7 +498,7 @@ You can combine all three options with each other and with the normal file argum
 Comparing Snapshots
 *******************
 
-Restic has a `diff` command which shows the difference between two snapshots
+Restic has a ``diff`` command which shows the difference between two snapshots
 and displays a small statistic, just pass the command two snapshot IDs:
 
 .. code-block:: console
@@ -506,9 +506,9 @@ and displays a small statistic, just pass the command two snapshot IDs:
     $ restic -r /srv/restic-repo diff 5845b002 2ab627a6
     comparing snapshot ea657ce5 to 2ab627a6:
 
-     C   /restic/cmd_diff.go
+     M   /restic/cmd_diff.go
     +    /restic/foo
-     C   /restic/restic
+     M   /restic/restic
 
     Files:           0 new,     0 removed,     2 changed
     Dirs:            1 new,     0 removed
@@ -527,6 +527,24 @@ folder, you could use the following command:
 
     $ restic -r /srv/restic-repo diff 5845b002:/restic 2ab627a6:/restic
 
+By default, the ``diff`` command only lists differences in file contents.
+The flag `--metadata` shows changes to file metadata, too.
+
+The characters left of the file path show what has changed for this file:
+
++-------+-----------------------+
+| ``+`` | added                 |
++-------+-----------------------+
+| ``-`` | removed               |
++-------+-----------------------+
+| ``T`` | entry type changed    |
++-------+-----------------------+
+| ``M`` | file content changed  |
++-------+-----------------------+
+| ``U`` | metadata changed      |
++-------+-----------------------+
+| ``?`` | bitrot detected       |
++-------+-----------------------+
 
 Backing up special items and metadata
 *************************************