Improve API documentation for fields

[bcorrie]

A user has pointed out that our documentation for API query fields is not particular good. We don't actually have a list of fields that the user can use to query the API as the GDC docs do. https://docs.gdc.cancer.gov/API/Users_Guide/Appendix_A_Available_Fields/#projects-field-groups

For example:

• It is hard for a user to determine that Diagnosis fields are subject.diagnosis.xxx
• It is hard for the user to determine that CellProcessing, NucleicAcidProcessing, and SequencingRun fields are denoted using the sample prefix (e.g. sample.cell_phenotype)

The hierarchy is essentially below - where FIELD(Study) means any Field from the Study object. But the user has to figure this out from reading the spec I believe 8-(

study.FIELD(Study) subject.FIELD(Subject) subject.diagnosis.FIELD(Diagnosis) sample.FIELD(Sample) sample.FIELD(CellProcessing) sample.FIELD(NucleicAcidProcessing) sample.FIELD(SequencingRun) sample.pcr_target.FIELD(PCRTarget) sample.sequencing_files.FIELD(SequencingData) data_processing.FIELD(DataProcessing)

[bcorrie]

This is worth keeping as something to do, but we won't get there for v2.0.