could you suppot some detail documentation for the method "preload"?

**Glide Version**:4.9.0

**Integration libraries**:

**Device/Android Version**:

**Issue details / Repro steps / Use case background**: i want to preload the image into the cache being sure the image will load quickly in the next time。And there is a question,is the method pereload() running in the background thread?And how to use preload()。

**Glide load line / `GlideModule` (if any) / list Adapter code (if any)**:
```java
Glide.with(Application).load(url).preload();
```

**Layout XML**:
```xml