Fix DataFrame.js comments for jsdoc

- Delete :::experimental:::
- Delete /* @scala.annotation.varargs */
- Add @example tags as appropriate (usually in replacement of {{{...}}} blocks)
- Fill in @param tags where missing.
- Convert [[[..]]] blocks to {@link ..}
- Complete exising @param tags. Notably missing are annotations for
optional parameters. No need to fill in type annotations at this
point (I'm hoping there's a tool that will allow extraction of
the flow type annotations...).