update documentation on env vars
The wiki has some information about GIO env vars:
https://xgitlab.cels.anl.gov/hacc/genericio/-/wikis/home
But I do not think it is complete, eg:
When run with GENERICIO_RANK0_CREATE_ALL=1, rank 0 creates all files (including subfiles) (https://xgitlab.cels.anl.gov/hacc/genericio/commit/f66a4a0ff9b3928ffb77a95f70cf9a449e6da3e7
We should keep a running list of all GIO env vars with descriptions/examples.