sql: elaborate more on optimizer hints (#2573)
sql: elaborate more on optimizer hints (#2573)

* elaborate more on optimizer hints

* Apply suggestions from code review

Co-authored-by: Null not nil <67ba71cae9aacd103ea348226341ecaf875f3948@users.noreply.github.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>

Co-authored-by: Null not nil <67ba71cae9aacd103ea348226341ecaf875f3948@users.noreply.github.com>
Co-authored-by: TomShawn <3d0966147538bc5ab020e8dffa2c0b746f9728a6@users.noreply.github.com>
pingcap/docs,pingcap/docs,pingcap/docs