message: Add RustDoc to `span::Data` methods
diff: use super::{DebugFields, Dispatcher, StaticMeta, Subscriber, Value};
  use std::{
  - borrow::Borrow,
  cell::RefCell,
  cmp, fmt,
  hash::{Hash, Hasher},
  @@ -324,26 +323,34 @@ impl Data {
  }
  }
  + /// Returns the name of this span, or `None` if it is unnamed,
  pub fn name(&self) -> Option<&'static str> {
  self.inner.name
  }
  + /// Returns a `Data` reference to the parent of this span, if one exists.
  pub fn parent(&self) -> Option<&Data> {
  self.inner.parent.as_ref()
  }
  + /// Borrows this span's metadata.
  pub fn meta(&self) -> &'static StaticMeta {
  self.inner.static_meta
  }
  + /// Returns an iterator over the names of all the fields on this span.
  pub fn field_names<'a>(&self) -> slice::Iter<&'a str> {
  self.inner.static_meta.field_names.iter()
  }
  + /// Returns the `Instant` at which this span was created.
  pub fn opened_at(&self) -> Instant {
  self.inner.opened_at
  }
  +
  + /// Borrows the value of the field named `name`, if it exists. Otherwise,
  + /// returns `None`.
  pub fn field<Q>(&self, key: Q) -> Option<&dyn Value>
  where
  &'static str: PartialEq<Q>,
  @@ -356,16 +363,20 @@ impl Data {
  .map(AsRef::as_ref))
  }
  + /// Returns an iterator over all the field names and values on this span.
  pub fn fields<'a>(&'a self) -> impl Iterator<Item = (&'a str, &'a dyn Value)> {
  self.field_names()
  .enumerate()
  .map(move |(idx, &name)| (name, self.inner.field_values[idx].as_ref()))
  }
  + /// Returns a struct that can be used to format all the fields on this
  + /// span ith `fmt::Debug`.
  pub fn debug_fields<'a>(&'a self) -> DebugFields<'a, Self> {
  DebugFields(self)
  }
  + /// Returns the current [`State`] of this span.
  pub fn state(&self) -> State {
  match self.inner.state.load(Ordering::Acquire) {
  s if s == State::Unentered as usize => State::Unentered,
  