deny(missing_docs) for bevy_diagnostic (#19482)

# Objective

Deny missing docs in bevy_diagnostic, towards
https://github.com/bevyengine/bevy/issues/3492.

crates/bevy_diagnostic/src/entity_count_diagnostics_plugin.rs

@@ -19,8 +19,10 @@ impl Plugin for EntityCountDiagnosticsPlugin {
 }
 
 impl EntityCountDiagnosticsPlugin {
+    /// Number of currently allocated entities.
     pub const ENTITY_COUNT: DiagnosticPath = DiagnosticPath::const_new("entity_count");
 
+    /// Updates entity count measurement.
     pub fn diagnostic_system(mut diagnostics: Diagnostics, entities: &Entities) {
         diagnostics.add_measurement(&Self::ENTITY_COUNT, || entities.len() as f64);
     }

crates/bevy_diagnostic/src/frame_time_diagnostics_plugin.rs

@@ -58,10 +58,16 @@ impl Plugin for FrameTimeDiagnosticsPlugin {
 }
 
 impl FrameTimeDiagnosticsPlugin {
+    /// Frames per second.
     pub const FPS: DiagnosticPath = DiagnosticPath::const_new("fps");
+
+    /// Total frames since application start.
     pub const FRAME_COUNT: DiagnosticPath = DiagnosticPath::const_new("frame_count");
+
+    /// Frame time in ms.
     pub const FRAME_TIME: DiagnosticPath = DiagnosticPath::const_new("frame_time");
 
+    /// Updates frame count, frame time and fps measurements.
     pub fn diagnostic_system(
         mut diagnostics: Diagnostics,
         time: Res<Time<Real>>,

crates/bevy_diagnostic/src/lib.rs

@@ -1,4 +1,3 @@
-#![expect(missing_docs, reason = "Not all docs are written yet, see #3492.")]
 #![cfg_attr(docsrs, feature(doc_auto_cfg))]
 #![forbid(unsafe_code)]
 #![doc(