Update Descriptor Guidance

The current descriptor guidance does not capture the recent community direction. For example, the direction proposed in the Technical Advisory Group and meetings such as: 

https://techdocs.ed-fi.org/display/EE/Managing+Descriptors%2C+Setting+Expectations+and+Certification+Implications 

It needs to be updated to capture the recommendation of when/how to use more local values and the capturing of canonical mappings to other contexts.