Add extra_properties config documentation

The configuration documentation table was not added for the
`PropertySpelling` linter.

Change-Id: I2bd61c88d0e78f302dd75858129b6c5d1a03f99f
Reviewed-on: https://gerrit.causes.com/33678
Tested-by: jenkins <jenkins@causes.com>
Reviewed-by: Shane da Silva <shane@causes.com>