Clarify mutually exclusive options in span creation. (#131)

* Clarify mutually exclusive options in span creation.

* Fix indentation.

* Add a note about parameters being language-specific.

specification/tracing-api.md

@@ -213,6 +213,16 @@ Optional parameters (or corresponding setters on `Builder` if using a `Builder`
 - The option to become a root `Span` for a new trace.
   If not set, the value of [Tracer.getCurrentSpan](#getcurrentspan) at `StartSpan`
   time will be used as parent.
+
+- **Note**: The three parameters above (parent `Span`, parent `SpanContext` and `root`) are
+  mutually exclusive. Based on language implementation, if multiple parameters are specified
+  or corresponding `Setter`s are called multiple times, only the last specified value will be used.
+  For example:
+    1. `builder.setParent(parentSpan).setNoParent().startSpan()` will generate a new root span
+    and `parentSpan` will be ignored;
+    2. `tracer.StartSpan(options.WithNoParent(), options.WithParentContext(parentCtx))`
+    will generate a new child span with remote parent `parentCtx`, and `WithNoParent` will be ignored.
+
 - `Sampler` to the newly created `Span`. If not set, the implementation should provide a
   default sampler used by Tracer.
 - Collection of `Link`s that will be associated with the newly created Span