In this section, the manual says "To unplug a PDB, connect to the root and use the ALTER PLUGGABLE DATABASE statement"
However, on the next page, under The following prerequisites must be met:, it says: "The current user must have SYSDBA or SYSOPER administrative privilege, and the privilege must be either commonly granted or locally granted in the PDB. The user must exercise the privilege using AS SYSDBA or AS SYSOPER at connect time."
If you have to be connected to the root, I am pretty sure that you can't be using a SYSDBA/SYSOPER privilege granted in the PDB.
These same basic statements are repeated in the following section on Dropping a PDB.