Document an interface

Signed-off-by: Andres Taylor <antaylor@squareup.com>

go/trace/trace.go

@@ -89,8 +89,13 @@ func CopySpan(parentCtx, spanCtx context.Context) context.Context {
 
 // SpanFactory is an interface for creating spans or extracting them from Contexts.
 type SpanFactory interface {
+	// New creates a new span from an existing one, if provided. The parent can also be nil
 	New(parent Span, label string, spanType SpanType) Span
+
+	// Extracts a span from a context, making it possible to annotate the span with additional information.
 	FromContext(ctx context.Context) (Span, bool)
+
+	// Creates a new context containing the provided span
 	NewContext(parent context.Context, span Span) context.Context
 }