doc: more about signals (sigh_storage_mixin)

docs/md/entity.md

@@ -15,6 +15,7 @@
 * [Storage](#storage)
 * [The Registry, the Entity and the Component](#the-registry-the-entity-and-the-component)
   * [Observe changes](#observe-changes)
+    * [Intent versus outcome](#intent-versus-outcome)
     * [Listeners disconnection](#listeners-disconnection)
     * [They call me Reactive System](#they-call-me-reactive-system)
   * [Sorting: is it possible?](#sorting-is-it-possible)
@@ -409,6 +410,38 @@ There are many useful but less known functionalities that aren't described here,
 such as the connection objects or the possibility to attach listeners with a
 list of parameters that is shorter than that of the signal itself.
 
+### Intent versus outcome
+
+The basic implementation of signal support (offered via storage mixin) doesn't
+account for the fact that _opaque_ creation or copying of components can fail if
+their types aren't default constructible or copyable.<br/>
+The mixin sends a signal to the listeners whenever the _intention_ to build a
+component is manifested and after this operation has occurred, even in the event
+of failure.
+
+Opaque creation and copying occurs when the caller interacts with a storage via
+its base class, without actually knowing what type of component it's working
+with:
+
+```cpp
+using namespace entt::literals;
+auto *storage = registry.storage("velocity"_hs);
+
+if(storage && (storage->emplace(entity) != storage->end())) {
+    // ...
+}
+```
+
+In this case, the operation may fail at runtime rather than compile-time and
+therefore the entity ends up without the desired component.<br/>
+The user can detect the error immediately by checking the returned iterator.
+However, the signal is sent anyway.
+
+In general, users should make component types default constructible and copyable
+regardless.<br/>
+However, it's useful to be aware of this eventuality in the event of errors and
+to act accordingly.
+
 ### Listeners disconnection
 
 The destruction order of the storage classes and therefore the disconnection of