clarify that showquota won't work on compute nodes
https://docs.nersc.gov/filesystems/quotas/#showquota is worded ambiguously; should be updated to "Perlmutter login nodes and Data Transfer Nodes"
https://docs.nersc.gov/filesystems/quotas/#showquota is worded ambiguously; should be updated to "Perlmutter login nodes and Data Transfer Nodes"