subscriber: "implementing FormatEvent" docs (#1727)

This branch adds some documentation to the `FormatEvent` trait in
`tracing_subscriber::fmt` on how to write user-provided `FormatEvent`
implementations. There's probably room for additional improvement here,
but I just wanted to get something written down for now.

I also fixed a broken link I noticed while I was here.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>
Co-authored-by: David Barsky <me@davidbarsky.com>

tracing-subscriber/src/fmt/format/mod.rs

@@ -155,7 +155,7 @@ pub use pretty::*;
 ///
 /// [`fmt::Layer`]: super::Layer
 /// [`fmt::Subscriber`]: super::Subscriber
-/// [`layer::Context`]: crate::layer::Context
+/// [`Event`]: tracing::Event
 /// [implements `FormatFields`]: super::FmtContext#impl-FormatFields<'writer>
 /// [ANSI terminal escape codes]: https://en.wikipedia.org/wiki/ANSI_escape_code
 /// [`Writer::has_ansi_escapes`]: Writer::has_ansi_escapes