Update 'grades' docs

The component grades docs at
http://wiki.fluidproject.org/display/docs/Component+Grades
need to be updated to include information about

• defining custom grades
• combining multiple grades, grade merging

plus

• dynamic grades, both supplied via direct options as well as those resolved via an IoC reference in the "gradeNames" list - explain the limitations on this latter process and how it must be used carefully