Document how "changed" breakpoints affect subsequent "setBreakpoints" requests #542

If a client sends a breakpoint on line 1 column 4, but then the DA resolves the breakpoint to line 2 column 1 (and sends a "changed" event), it's not currently specified whether adding more breakpoints to the same file should result in the original breakpoint being sent again as 1:4, or as 2:1.

I filed this as a bug in VS Code #545 (because I feel like it should be updated), however I feel the spec should be clear about this regardless of whether VS Code's behaviour is what's intended or not.

Since breakpoints are always sent as a complete list and a DA might want to just remove deleted breakpoints and add new ones, it needs to be able to match up the breakpoints with those that already exist, and it needs to know whether to do that using the "current" locations of the breakpoints (that it has resolved), or the "original" locations sent by the client. If clients don't behave the same, then this optimisation will not work the same for all clients.