Reword delete-orphan on many error message and document

Reword delete-orphan on many error message and document

For many years we have encountered users making use of the
"single_parent" flag in response to the error message for
"delete-orphan" expressing this as a means to cancel the current
error.   However, the actual issue here is usually a misuse
of the delete-orphan cascade setting.  Reword the error message to
be much more descriptive about what this means and add new
error link sections describing the situation in as much detail
as possible.

Fixes: #5329
Change-Id: I7ba710378b2935479ab22ff9a0a79c692dbf69a6