message: docs: add docs for instrument(name)
  ## Motivation
  Not found in documentation how to change generated span name name, only in
  sources, then i found issue
  ## Solution
  Documentation added
diff: @@ -121,6 +121,14 @@ use syn::{
  /// // ...
  /// }
  /// ```
  +/// Overriding the generated span's name:
  +/// ```
  +/// # use tracing_attributes::instrument;
  +/// #[instrument(name = "my_name")]
  +/// pub fn my_function() {
  +/// // ...
  +/// }
  +/// ```
  /// Overriding the generated span's target:
  /// ```
  /// # use tracing_attributes::instrument;
  