[harmony] [docs] variables and subsetting

[JessicaS11]

We currently emphasize icepyx' ability to request variable-subsetted data because it can substantially decrease the file size. A non-exhaustive list of places we'll need to address this transition includes:

• [ ] docstrings
• [ ] general documentation
• [ ] examples: data access
• [ ] examples: variable subsetting
• [ ] other examples that link to the above