docs: Document stub/not-implemented methods in resources module

Fixed discrepancies between source code and documentation:
- AsyncLoader: Document Initialize() ignores workerThreadCount, Submit() doesn't do actual async loading, Update() is stub
- ResourceManager: Document UnloadUnused() and ReloadResource() are stubs
- ResourceCache: Document OnZeroRefCount() and Flush() are stubs
- ResourceDependencyGraph: Document TopologicalSort() returns empty (stub)
- ResourceFileSystem: Document GetResourceInfo() doesn't fill modifiedTime, EnumerateResources() is stub
- FileArchive: Document Enumerate() is stub
- ResourcePackageBuilder: Document AddDirectory() is stub
- ImportSettings: Document LoadFromJSON/SaveToJSON are stubs
- TextureImportSettings/MeshImportSettings: Document JSON methods are stubs
- TextureLoader/MeshLoader/MaterialLoader/ShaderLoader/AudioLoader: Document GetDefaultSettings() returns nullptr
- AudioLoader: Document ParseWAVData() is stub, Load() doesn't parse WAV headers
- ShaderLoader: Document DetectShaderType/ParseShaderSource are stubs
- MaterialLoader: Document ParseMaterialData() is stub
- Texture: Document Create() mipLevels=0 behavior, GenerateMipmaps() returns false
- Mesh: Document MeshLoader::Load() is example only
- IResourceLoader: Document GetDefaultSettings() returns nullptr for all loaders

docs/api/resources/shader/shader.md

@@ -79,6 +79,10 @@ Uniform 变量描述。
 | `size_t GetMemorySize() const` | 获取内存大小 |
 | `void Release()` | 释放着色器引用 |
 
+## 实现说明
+
+**注意**: `ShaderLoader::DetectShaderType()` 对于非标准扩展名始终返回 `ShaderType::Fragment`。`ShaderLoader::ParseShaderSource()` 为 stub，始终返回 true。
+
 ### 类型与语言
 
 | 方法 | 描述 |