message: core: fix incorrect docs in `tracing_core::LevelFilter`
  Resolves Namely, I:
  fixed the incorrect docs on `LevelFilter`.
  Removed a stray backtick on `LevelFilter::current`.
  Added a matching backtick in Level's documentation.
  I did _not_ add example of comparing a level against a `LevelFilter`; I
  instead pointed readers to Level's documentation.
diff: @@ -128,7 +128,7 @@ pub struct Kind(KindInner);
  /// verbosity. `Level`s can be compared against [`LevelFilter`]s, and
  /// [`LevelFilter`] has a variant for each `Level`, which compares analogously
  /// to that level. In addition, [`LevelFilter`] adds a [`LevelFilter::OFF`]
  -/// variant, which is considered "less verbose" than every other `Level. This is
  +/// variant, which is considered "less verbose" than every other `Level`. This is
  /// intended to allow filters to completely disable tracing in a particular context.
  ///
  /// For example:
  @@ -227,14 +227,16 @@ pub struct Level(LevelInner);
  /// A filter comparable to a verbosity [`Level`].
  ///
  /// If a [`Level`] is considered less than a `LevelFilter`, it should be
  -/// considered disabled; if greater than or equal to the `LevelFilter`, that
  -/// level is enabled.
  +/// considered enabled; if greater than or equal to the `LevelFilter`,
  +/// that level is disabled. See [`LevelFilter::current`] for more
  +/// details.
  ///
  /// Note that this is essentially identical to the `Level` type, but with the
  /// addition of an [`OFF`] level that completely disables all trace
  /// instrumentation.
  ///
  -/// See the documentation for the [`Level`] type for more details.
  +/// See the documentation for the [`Level`] type to see how `Level`s
  +/// and `LevelFilter`s interact.
  ///
  /// [`OFF`]: LevelFilter::OFF
  #[repr(transparent)]
  @@ -584,7 +586,7 @@ impl LevelFilter {
  /// Therefore, comparing a given span or event's level to the returned
  /// `LevelFilter` **can** be used for determining if something is
  /// *disabled*, but **should not** be used for determining if something is
  - /// *enabled*.`
  + /// *enabled*.
  ///
  /// [`Level`]: super::Level
  /// [collector]: super::Collect
  