subscriber: add more cross-references to documentation (tokio-rs#1553)

This branch adds some new documentation in a few places that references
other sections in the docs. This should help make some of the crate's
APIs easier to find. In particular, the crate's top level docs now
summarize the `Layer` and `Filter` traits, and link to their individual
docs sections. Additionally, the per-layer filtering section in the
`Layer` module docs now references some of the concrete `Filter`
implementations in the `filter` module.

tracing-subscriber/src/layer/mod.rs

@@ -202,6 +202,14 @@
 //! potentially record them. The [`Layer::with_filter`] method combines a
 //! `Layer` with a [`Filter`], returning a [`Filtered`] layer.
 //!
+//! This crate's [`filter`] module provides a number of types which implement
+//! the [`Filter`] trait, such as [`LevelFilter`], [`Targets`], and
+//! [`FilterFn`]. These [`Filter`]s provide ready-made implementations of
+//! common forms of filtering. For custom filtering policies, the [`FilterFn`]
+//! and [`DynFilterFn`] types allow implementing a [`Filter`] with a closure or
+//! function pointer. In addition, when more control is required, the [`Filter`]
+//! trait may also be implemented for user-defined types.
+//!
 //! <div class="example-wrap" style="display:inline-block">
 //! <pre class="compile_fail" style="white-space:normal;font:inherit;">
 //!     <strong>Warning</strong>: Currently, the <a href="../struct.Registry.html">
@@ -396,6 +404,10 @@
 //! [`Layer::enabled`]: Layer::enabled
 //! [`Interest::never()`]: https://docs.rs/tracing-core/latest/tracing_core/subscriber/struct.Interest.html#method.never
 //! [`Filtered`]: crate::filter::Filtered
+//! [`filter`]: crate::filter
+//! [`Targets`]: crate::filter::Targets
+//! [`FilterFn`]: crate::filter::FilterFn
+//! [`DynFilterFn`]: crate::filter::DynFilterFn
 //! [level]: tracing_core::Level
 //! [`INFO`]: tracing_core::Level::INFO
 //! [`DEBUG`]: tracing_core::Level::DEBUG