Clarify that PD gets created, it's not automatically there

[avahoffman]

Data_on_AnVIL/02-bringing_own_data.Rmd

Line 133 in b71b7af

Terra workspaces have two dedicated storage locations: the workspace bucket and the Cloud Environment virtual machine (VM) Persistent Disk (PD). Like a USB drive, the PD can be detached from the VM before deleting or recreating the Cloud Environment, then attached to a new one. The PD lets you keep your notebook's code packages, input files, and outputs - without having to move anything to workspace storage (i.e., Google Bucket).