Clean up constants so that flags, pixel formats, etc. are clearly differentiated...

Clean up constants so that flags, pixel formats, etc. are clearly differentiated; Update documentation accordingly; Name the enums to make it easier to reference them in the docs and clean up the references accordingly; Set Doxygen option to force a detailed description for the constants to always be generated

git-svn-id: svn+ssh://svn.code.sf.net/p/libjpeg-turbo/code/trunk@617 632fc199-4ca6-4c93-a231-07263d6284db