Provide brief documentation for all cta-admin commands

Some cta-admin commands are currently documented, for example tapefile:

cta-admin tf
Missing subcommand
cta-admin tf/tapefile ls/rm:

  Tape files can be listed by VID or by EOS disk instance + EOS disk file ID.
  Disk file IDs should be provided in hexadecimal (fxid). The --fxidfile option
  takes a file in the same format as the output of 'eos find --fid <path>'
  Delete a file copy with the "rm" subcommand

    ls [--vid/-v <vid>] [--instance/-i <disk_instance>] [--fxid/-f <eos_fxid>]
       [--fxidfile/-F <filename>] [--id/-I <archive_file_id>]
    rm --vid/-v <vid> [--instance/-i <disk_instance>] [--fxid/-f <eos_fxid>]
       [--id/-I <archive_file_id>] --reason/-r <reason_status_change>

However there are many others which are not, for example physicallibrary, logicallibrary. We should add a short description for all cta-admin commands.