Add documentation of using redef to redefine initial value of options.
Mention caveats for changing the value of specific data types.
Show an example of how to use the Config::set_value() function.
Other small improvements to the examples and text.
Includes slight editing of the new text.
Closes BIT-1905.
* origin/topic/dnthayer/ticket1905:
Add documentation of the configuration framework
Add documentation of "option" declarations
Improve config framework documentation comments
Fix minor typos and doc build warnings in NEWS
