Update descriptor guidance

Data Standard 4.0 will introduce new options for descriptor management and exchange, particularly via the DescriptorMapping entity. These new options need to be explained, and also likely change the guidance given currently - for example, in this document https://techdocs.ed-fi.org/display/EFDS33/Descriptor+Guidance 

Some of the new general principles, coming out of community engagement from the TAG, Technical Congress 2022 and elsewhere:

• systems with multiple mappings relevant should send data in the most "local" format and provide a mapping to other contexts. A good example is providing data from a SIS to a LEA
• Agencies should consider the data exchange context and the source system values when considering what descriptors to use. For example, if the goal is to provide data for evaluation of local programs, local codes will be the most valuable descriptors to provide, and may even be required
• As the above case makes clear, the Ed-Fi values represent default, suggested values only; agencies using Ed-Fi should not see Ed-Fi values as canonical