Add the description of `--sort-files` option to the man pages of `ltfs_ordered_copy`

[piste-jp]

Is your feature request related to a problem? Please describe.

--sort-files option was introduced into #512. But man page description is not included into the PR. So need to add it.

Describe the solution you'd like

--sort-files=<true|false>: make alphabetical sort to the source file list if they are on disk. (default: true)