Docs: docstring of verdi group delete command can be confusing

[ltalirz]

This is the current docstring of verdi group delete:

aiida-core/aiida/cmdline/commands/cmd_group.py

Lines 66 to 71 in 5e02e16

	"""Delete a group.
	Note that a group that contains nodes cannot be deleted if it contains any nodes. If you still want to delete the
	group, use the `-c/--clear` flag to remove the contents before deletion. Note that in any case, the nodes themselves
	will not actually be deleted from the database.
	"""

The differentiation between a node being removed from a group and a node being deleted is unclear.
Furthermore, the docstring is inconsistent in itself & contains unnecessary repetitions.

Will provide a suggestion for improvement.

[ltalirz]

this was fixed in #4357