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


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@385048 65c4cc65-6c06-0410-ace0-fbb531ad65f3

configs/extconfig.conf.sample

@@ -58,13 +58,17 @@
 ; start at 1 and be sequential (i.e. if you have only priorities 1, 2,
 ; and 4, then 4 will be ignored, because there is no 3).
 ;
+;
+; Possible driver backends:
+;
 ; "odbc" is shown in the examples below, but is not the only valid realtime
-; engine.  There is:
+; engine.  Here are several of the possible options:
 ;    odbc ... res_config_odbc
 ;    sqlite ... res_config_sqlite
 ;    pgsql ... res_config_pgsql
 ;    curl ... res_config_curl
 ;    ldap ... res_config_ldap
+;    mysql ... res_config_mysql (available via add-ons in menuselect)
 ;
 ; Note: The res_config_pgsql and res_config_sqlite backends configure the
 ; database used in their respective configuration files and ignore the