docs: different pattern matching for --exclude, fixes #1779

docs/usage.rst

@@ -387,6 +387,15 @@ Examples
     # As above, but add nanoseconds
     $ borg create /path/to/repo::{hostname}-{user}-{now:%Y-%m-%dT%H:%M:%S.%f} ~
 
+Notes
+~~~~~
+
+- the --exclude patterns are not like tar. In tar --exclude .bundler/gems will
+  exclude foo/.bundler/gems. In borg it will not, you need to use --exclude
+  '*/.bundler/gems' to get the same effect. See ``borg help patterns`` for
+  more information.
+
+
 .. include:: usage/extract.rst.inc
 
 Examples