core: update parent() documentation (#1665)

`Event::parent()` and `Span::parent()` returns an `Option`, not boolean.

Author: mbergkvist

tracing-core/src/event.rs

@@ -118,7 +118,7 @@ impl<'a> Event<'a> {
     /// Returns the new event's explicitly-specified parent, if there is one.
     ///
     /// Otherwise (if the new event is a root or is a child of the current span),
-    /// returns false.
+    /// returns `None`.
     pub fn parent(&self) -> Option<&Id> {
         match self.parent {
             Parent::Explicit(ref p) => Some(p),

tracing-core/src/span.rs

@@ -168,7 +168,7 @@ impl<'a> Attributes<'a> {
     /// Returns the new span's explicitly-specified parent, if there is one.
     ///
     /// Otherwise (if the new span is a root or is a child of the current span),
-    /// returns false.
+    /// returns `None`.
     pub fn parent(&self) -> Option<&Id> {
         match self.parent {
             Parent::Explicit(ref p) => Some(p),