Add documentation for HttpException

Corresponds with https://github.com/tachiyomiorg/extensions-lib/pull/12

core/src/main/java/eu/kanade/tachiyomi/network/OkHttpExtensions.kt

@@ -143,4 +143,11 @@ fun <T> decodeFromJsonResponse(
     }
 }
 
+/**
+ * Exception that handles HTTP codes considered not successful by OkHttp.
+ * Use it to have a standardized error message in the app across the extensions.
+ *
+ * @since extensions-lib 1.5
+ * @param code [Int] the HTTP status code
+ */
 class HttpException(val code: Int) : IllegalStateException("HTTP error $code")