Mariabackup's usage message for --use-memory option requires clarification

Mariabackup's --use-memory option is only relevant when performing a --prepare. However, the usage message does not mention this:

$ mariabackup --help | grep use-memory

  --use-memory=#      The value is used instead of buffer_pool_size

It should probably be clarified, so it says something like this instead:

The value is used in place of innodb_buffer_pool_size. This option is only relevant when the --prepare option is specified.