docs: datetime formatting examples for {now} placeholder, fixes #1822

docs/usage.rst

@@ -382,7 +382,10 @@ Examples
 
     # Use short hostname, user name and current time in archive name
     $ borg create /path/to/repo::{hostname}-{user}-{now} ~
-    $ borg create /path/to/repo::{hostname}-{user}-{now:%Y-%m-%d_%H:%M:%S} ~
+    # Similar, use the same datetime format as borg 1.1 will have as default
+    $ borg create /path/to/repo::{hostname}-{user}-{now:%Y-%m-%dT%H:%M:%S} ~
+    # As above, but add nanoseconds
+    $ borg create /path/to/repo::{hostname}-{user}-{now:%Y-%m-%dT%H:%M:%S.%f} ~
 
 .. include:: usage/extract.rst.inc