Auto merge of #10941 - hi-rustin:rustin-patch-docs, r=weihanglo

Auto merge of #10941 - hi-rustin:rustin-patch-docs, r=weihanglo

Mark --timings=html unstable in the document

### What does this PR try to resolve?

Mark -timings=html unstable in the document. The document was confusing because if you try `--timings=html` it still requires the unstable flag.

### How should we test and review this PR?

Check out the document.

### Additional information

Other information you want to mention in this PR, such as prior arts,
future extensions, an unresolved problem, or a TODO list.