Clarify the help for "breakpoint command add" and "watchpoint command add".

These two commands add a list of commands to the breakpoint/watchpoint. The current
implementation only supports replacing the current command list.  I started with
that as overwrite seems to be the most common operation.  But using "add" will
allow us to later offer other add-modes: "prepend", "append" and "insert".
That and "overwrite" then make up a useful set of options for this operation.