[BUGFIX][INSTALL] Show description for enableDeprecationLog as <dl>

Currently it's listed inline in the text, make it a definition list.

Change-Id: I18c2df10a0e18dbc167b40465192909cf30278e6
Resolves: #28024
Reviewed-on: http://review.typo3.org/3174
Reviewed-by: Philipp Gampe
Reviewed-by: Stefan Neufeind
Reviewed-by: Xavier Perseguers
Reviewed-by: Benjamin Mack
Tested-by: Benjamin Mack