Clarify behavior of `Cancel` and `BreakpointLocations` request

[DrSergei]

For Cancel and BreakpointLocations requests arguments field could be omitted according to DAP specification. It looks like VS Code always sends these requests with correct arguments, so it might be a typo in the specification. What is expected behavior in this case?

[tromey]

BreakpointLocations was previously discussed in #266