[FEATURE] Add description-field for filemounts

[FEATURE] Add description-field for filemounts

With the new field it is possible to give a short description what a
certain filemount should be used for or which documents they contain, etc.

Change-Id: Ia0e9d8385d8a6f0620b20581f4acc497dc9ffca9
Resolves: #59570
Releases: master
Reviewed-on: http://review.typo3.org/40285
Reviewed-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Tested-by: Mathias Schreiber <mathias.schreiber@wmdb.de>
Reviewed-by: Felix Kopp <felix-source@phorax.com>
Tested-by: Felix Kopp <felix-source@phorax.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>