Documenting the recently added 'ceph tell <pgid> scrub-abort'
command.
As this is the first command using the 'tell <pgid>' syntax
that is documented, also adding the two other scrub-related ``tell``
commands.
Note that the 'tell <pgid> scrub-abort' is the only available
format of this specific command. The other two commands
also support the 'tell pg <cmd> <pgid>' format.