Modified the list of keys for the driver backends for sake of sample clarity

Added a line showing the mapping of "mysql" to res_config_mysql available in add-ons. We used "mysql" as an example driver key in the sample, but didn't show what module it mapped too. Also added a subtitle above the list of keys for driver backends.
........

Merged revisions 385047 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/11@385048 f38db490-d61c-443f-a65b-d21fe96a405b