Add descriptions (and help text where necessary) to all membership-related custom objects and fields

[eramirez-arkus]

Descriptions and help text are being drafted as part of the March 2023 Sprint (documentation group)

See: https://docs.google.com/spreadsheets/d/1B1I9xvZy-TzkVSjIY2hMOYKR_Nkt3lSLpYklFZbz8ys/edit#gid=0

[JLrlCap]

I'd be happy to take this on once we're ready to add these to the org.

[JLrlCap]

Didn't mean to close it...