Document limitation around Counter::absolute etc.

src/lib.rs

@@ -1,7 +1,14 @@
 //! A [`metrics`] exporter that supports reporting metrics to Statsd. This exporter is basically
 //! a thin wrapper on top of the [`cadence`] crate which supports Statsd/Datadog style metrics.
 //!
-//! **Versions of this crate are tightly coupled to metrics crate versions**
+//! # Warnings
+//!
+//! * **Versions of this crate are tightly coupled to metrics crate versions.**
+//!
+//! * [`metrics::Counter::absolute`], [`metrics::Gauge::increment`], and
+//!     [`metrics::Gauge::decrement`] are not supported. Statsd doesn't have these concepts.
+//!     Unfortunately this means that if the application is using these methods, the metrics will
+//!     silently be missing or wrong.
 //!
 //! # Usage
 //!