Fix HTML version of the keyword table.

[nodakai]

Really fix #17528.

Here is the resulting HTML output:

• https://dl.dropboxusercontent.com/u/107479221/reference.html#keywords

"Adjacent sibling selector" + was first introduced in CSS 2.1 and should be understood by many modern browsers.

[huonw]

It's possibly better to do this by inserting a (useless) thing just before the table, rather than relying on there being exactly one paragraph between the heading and the table, e.g.

<div id="keyword-table-marker"><!-- allow us to style the table specifically --></div>

|     |    | ...
|-----|...
...

and then the CSS would be #keyword-table-marker + table.

[nodakai]

@huonw I had some concern about possible side effects on PDF and ePUB outputs by Pandoc as I don't have Pandoc installed on my machine...

I'll use the paragraph saying "the following strings" itself as a marker because it almost guarantees the following element is the keyword table and I can expect less impact on Pandoc outputs than an empty DIV would do (if any.)

Let me also do piggybacking of some typographical changes. The result will look like this