Add comment about ulimit format.

Fixes #12116

Signed-off-by: David Calavera <david.calavera@gmail.com>

docs/sources/reference/commandline/cli.md

@@ -2399,6 +2399,9 @@ available in the default container, you can set these using the `--ulimit` flag.
 > In other words, the following script is not supported:
 > `$ docker run -it --ulimit as=1024 fedora /bin/bash`
 
+The values are sent to the appropriate `syscall` as they are set.
+Docker doesn't perform any byte conversion. Take this into account when setting the values.
+
 ## save
 
     Usage: docker save [OPTIONS] IMAGE [IMAGE...]