add note on "pander" and "asis"

When using the generic S3 method, the "asis" chunk option is not needed any more: http://blog.rapporter.net/2014/09/pander-tables-inside-of-knitr.html

This might be useful for someone working on the documentation, as IMHO it's not a good practice to call `pandoc.table` directly.

---

Although please feel free to close this pull request without merging if you do not think this comment is useful here.