Update documentation on jekyllrb.com

[matthewdu]

The previous doc about categories and tags does not seem to be true.

For

--- # Front Matter
...
categories: piano, music
...

---

and

--- # Front Matter
...
categories: "piano, music"
...

---

the parsed output for categories is

"categories"=>["piano,", "music"] # there's an extra comma

The same applies with tags

You can verify this by logging data at https://github.com/jekyll/jekyll/blob/master/lib/jekyll/document.rb#L480

Space-delimited string works though. Better yet, this should probably only say YAML list.

[ashmaroli]

/cc @jekyll/documentation

[ashmaroli]

👍

[DirtyF]

@jekyllbot: merge +site