document the logs-cache feature flag

Co-Authored-By: Eloi DEMOLIS <eloi.demolis@clever-cloud.com>

command/src/logging/mod.rs

@@ -1,3 +1,13 @@
+//! Sōzu logs, optimized for performance
+//!
+//! Instead of relying on well-known logging or tracing solutions,
+//! Sōzu has its own logging stack that prioritizes CPU performance
+//!
+//! The `logs-cache` flag, on of that, saves lookup time by storing
+//! the ENABLED status of each log call-site, in a `static mut`.
+//! The gain in performance is measurable with a lot of log directives,
+//! but mostly negligible, since CPUs are clever enough to recognize such patterns.
+
 pub mod access_logs;
 pub mod display;
 #[macro_use]