reference/performance: refine optimizer hints doc (#2093)
reference/performance: refine optimizer hints doc (#2093)

* reference/performance: refine optimizer hints doc

* make the given syntax and examples more user-friendly

* refine the content

* make three hints's alias info clearer

* update three notes

* Fix a query (confirmed with the author)

* resolve a comment

* address kenan's comments

* fix a typo

pingcap/docs-cn,pingcap/docs-cn