trace: fix the formatting of the package doc
The list is being rendered as one big sentence on godoc.
Adding new lines, so that godoc renders them one by one.
// about the span. The elements of this map are called labels. Some labels,
// whose keys all begin with the string "trace.cloud.google.com/", are set
// automatically in the following ways:
+//
// - SpanFromRequest sets some labels to data about the incoming request.
+//
// - NewRemoteChild sets some labels to data about the outgoing request.
+//
// - Finish sets a label to a stack trace, if the stack trace option is enabled
// in the incoming trace header.
+//
// - The WithResponse option sets some labels to data about a response.
// You can also set labels using SetLabel. If a label is given a value
// automatically and by SetLabel, the automatically-set value is used.
