add information about pool naming convention in cstor doc

[mynktl]

Naming convention of cstor pool is not mentioned in https://docs.openebs.io/docs/next/cstor.html and that may create confusion to user if he is not aware of zfs. Adding a section having information about how PV creation happens in cstor will be helpful for user to understand the flow and also internal of cstor. Same goes for PV deletion.

Refer: openebs/velero-plugin#51 (comment).