rsync_tmbackup.sh: Improved usage and output info

[Tormen]

... in order for it to be more intuitive and self-explanatory (user friendly for new users).

 * Added missing explanation about the exclude-from-file to make usage info (--help) self-contained to use for new users.
 * Renamed LOG_FILE to RSYNC_LOG_FILE (as LOG_FILE might be confused with the log of the output of this script)
 * Added the RSYNC_LOG_FILE path to the scripts output.
 * Clarified the difference between a FULL and an INCREMENTAL backup in the scripts output.
 * Forced a linebreak in script output (as line break was probable) to make things better readable.