DamageableSystem cleanup & performance improvements (#20820)

Content.Shared/Damage/DamageModifierSet.cs

@@ -5,11 +5,15 @@ using Robust.Shared.Serialization.TypeSerializers.Implementations.Custom.Prototy
 namespace Content.Shared.Damage
 {
     /// <summary>
-    ///     A set of coefficients or flat modifiers to damage types.. Can be applied to <see cref="DamageSpecifier"/> using <see
+    ///     A set of coefficients or flat modifiers to damage types. Can be applied to <see cref="DamageSpecifier"/> using <see
     ///     cref="DamageSpecifier.ApplyModifierSet(DamageSpecifier, DamageModifierSet)"/>. This can be done several times as the
     ///     <see cref="DamageSpecifier"/> is passed to it's final target. By default the receiving <see cref="DamageableComponent"/>, will
     ///     also apply it's own <see cref="DamageModifierSet"/>.
     /// </summary>
+    /// <remarks>
+    /// The modifier will only ever be applied to damage that is being dealt. Healing is unmodified.
+    /// The modifier can also never convert damage into healing.
+    /// </remarks>
     [DataDefinition]
     [Serializable, NetSerializable]
     [Virtual]