Add/improve xmldocs

osu.Framework/Graphics/Performance/LifetimeEntry.cs

···
 
 namespace osu.Framework.Graphics.Performance
 {
+    /// <summary>
+    /// An object for a <see cref="LifetimeEntryManager"/> to consume, which provides a <see cref="LifetimeStart"/> and <see cref="LifetimeEnd"/>.
+    /// </summary>
+    /// <remarks>
+    /// Management of the object which the <see cref="LifetimeEntry"/> refers to is left up to the consumer.
+    /// </remarks>
     public class LifetimeEntry
     {
         private double lifetimeStart = double.MinValue;
···
             }
         }
 
+        /// <summary>
+        /// Invoked when this <see cref="LifetimeEntry"/> is attached to a <see cref="LifetimeEntryManager"/> and either
+        /// <see cref="LifetimeStart"/> or <see cref="LifetimeEnd"/> are changed.
+        /// </summary>
         internal event RequestLifetimeUpdateDelegate RequestLifetimeUpdate;
 
         /// <summary>

osu.Framework/Graphics/Performance/LifetimeEntryState.cs

···
 
 namespace osu.Framework.Graphics.Performance
 {
+    /// <summary>
+    /// The state of a <see cref="LifetimeEntry"/>.
+    /// </summary>
     public enum LifetimeEntryState
     {
-        /// Not yet loaded.
+        /// <summary>
+        /// The <see cref="LifetimeEntry"/> hasn't been processed within the <see cref="LifetimeEntryManager"/> yet.
+        /// </summary>
         New,
 
-        /// Currently dead and becomes alive in the future: current time &lt; <see cref="Drawable.LifetimeStart"/>.
+        /// <summary>
+        /// The <see cref="LifetimeEntry"/> is currently dead and becomes alive when current time &gt;= <see cref="LifetimeEntry.LifetimeStart"/>.
+        /// </summary>
         Future,
 
-        /// Currently alive.
+        /// <summary>
+        /// The <see cref="LifetimeEntry"/> is currently alive.
+        /// </summary>
         Current,
 
-        /// Currently dead and becomes alive if the clock is rewound: <see cref="Drawable.LifetimeEnd"/> &lt;= current time.
+        /// <summary>
+        /// The <see cref="LifetimeEntry"/> is currently dead and becomes alive when current time &lt; <see cref="LifetimeEntry.LifetimeEnd"/>.
+        /// </summary>
         Past,
     }
 }