Add data documentation for new CMIP6 xray items

[brucecrevensten]

We know we need to review the documentation wrappers for the new CMIP6 items.

Be sure to document that 2010-2019 is omitted because it mixes data between model baseline and future. Consider making a follow-up ticket to fuse these more tightly but in practice that's been messy and it can be helpful to have this visual gap. TBD.

[brucecrevensten]

The intent of this ticket remains to evaluate the quality of the supporting documentation across all CMIP6 items. It's pretty generic "look, see what gaps / things are missing, address as possible." The GetAndUse segment is missing entirely here, for example.