FAQ. Add new item concerning unsafeCast & asDynamic (#2255)

* FAQ. Add new question & answer

* FAQ. Wrappers -> external declarations

* FAQ. Describe the purpose of `unsafeCast` and `asDynamic`

* FAQ. Define responsibility area for factories creation

* FAQ. Add a link for issue creation

* FAQ. Add a list for typical use cases. Blockquote for a opaque alias definition

* FAQ. & -> or

* FAQ. Fix link

* FAQ. Make an answer more consise

* FAQ. `or` -> `and`

* FAQ. `library author` -> `author of declarations`

* FAQ. `author of declarations` -> `declarations' owner`

* FAQ. Wording

FAQ.md

@@ -71,4 +71,30 @@ JS equivalent:
 const instance = {
   foo: "bar",
 }
-```
+```
+
+_Q_: Why should I avoid `unsafeCast` & `asDynamic` when working with Wrappers?
+_A_: First of all, `unsafeCast` and `asDynamic` are not recommended for external declarations, unless you are a declarations' owner. \
+Some types' instances cannot be created by constructor invocation or interface implementation. For such types, we
+provide a strict factory functions. It is a responsibility of library authors.
+Example:
+
+```kotlin
+sealed external interface ClassName
+
+inline fun ClassName(
+    value: String,
+): ClassName =
+    value.unsafeCast<ClassName>()
+
+val value = ClassName("my-class")
+```
+
+If there is no strict factory function for a type of this kind, please, [create an issue](https://github.com/JetBrains/kotlin-wrappers/issues).
+Otherwise, when you use `unsafeCast` or `asDynamic`, the type can be incorrect when a library updates.
+
+Typical use cases are: 
+- `JSO` (look at the previous item for JSO creation example)
+- Opaque alias
+> **Opaque alias** is the interface that mimicry another external interface but incapsulates some logic inside (e.g.,
+a `String` opaque interface without any string operations).