split long doc comment in approx_span (#538)

New clippy lint introduced in nightly (`clippy::too-long-first-doc-paragraph`) was being triggered on the top module comment for `result::approx_span`.

crates/rsonpath-lib/src/result/approx_span.rs

@@ -1,4 +1,5 @@
 //! [`Recorder`] implementation finding the starts and approximate ends of all matches.
+//!
 //! Faster than a full [`NodesRecorder`](super::nodes::NodesRecorder), but the span
 //! may include trailing whitespace after the actual matched value.
 use super::{output_queue::OutputQueue, *};