Create paragraph to say that Style Guide doesn't cover use of markup

[julian-cable]

[daobrien]

To help address this I'm going to rename the section 5.3. DocBook Elements for two reasons:

1. We've all but moved away from writing in DocBook, as has the product documentation team.
2. It's not really up to us to tell others how to use DocBook. Numerous online references exist that ppl can refer to.

I'll go with "Using Markup Correctly" or similar, but I don't want to get into a big discussion about how to use markup. I'll probably add something along the lines of "This is about writing, not markup."