The default behavior to add a newline break between the existing comment body and the new body is a good default.

I think it will be nice to have feature if we can disable the added break, or use a space instead. (Could be called append-separator with three options: newline, space, none.)

Example: If the original comment is:

Look out for an update here:

With append-separator: newline (default):

Look out for an update here:
This is the update.

With append-separator: space:

Look out for an update here: This is the update.

With append-separator: none:

Look out for an update here:This is the update.