documentation: Adds missing default attributes.

The configObject tag contains a default attribute which
allows the default value to be specified, if applicable.
This allows for the default value to show up specially on
the wiki in a way that is clear to users.

There are a couple places in the tree where default values
are included in the description as opposed to as attributes,
which means these can't be parsed specially for the wiki.
These are changed to use the attribute instead of being
included in the text description.

ASTERISK-29898 #close

Change-Id: I9d7ea08f50075f41459ea7b76654906b674ec755