Add SPARQL syntax sections on triple terms, reifiers, and annotations

[rubensworks]

The text and structure of these new sections are mostly based on the text in the Turtle 1.2 spec, with the necessary changes to the interpretation, grammar, and examples.

Closes #176
Closes #398

One discussion point I want to raise here (which I did not incorporate yet into this PR), is the following:
Similar to how a "Triple Term" is a "Triple" that can be placed in the object position, do we need a concept "Triple Term Pattern" to refer to a "Triple Pattern" that can be placed in the object position? IMO, the text in this PR works without such a concept, but perhaps someone else has a different view on it. If we do this, we may also have to do this for Reifying Triple (Patterns). We may also consider grammar renames for this.

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Preview | Diff

[kasei]

Are patterns "reified" or "asserted"? I think it's more natural to think of annotation syntax as representing patterns which match reified and asserted data.

[rubensworks]

Are patterns "reified" or "asserted"? I think it's more natural to think of annotation syntax as representing patterns which match reified and asserted data.

@kasei Good point. What do you think of the following?

Suggested change

	<p>SPARQL also defines an <dfn data-lt="annotation-syntax">annotation syntax</dfn>
	that both reifies and asserts a <a href="#defn_TriplePattern">triple pattern</a>,
	<p>SPARQL also defines an <dfn data-lt="annotation-syntax">annotation syntax</dfn>
	to represent <a href="#defn_TriplePattern">triple patterns</a> that match reified and asserted triples,

[TallTed]

I think this obscures the point of this sentence, that SPARQL's annotation syntax simultaneously reifies and asserts. Trying to fix that, starting with the edit above --

Suggested change

	<p>SPARQL also defines an <dfn data-lt="annotation-syntax">annotation syntax</dfn>
	that both reifies and asserts a <a href="#defn_TriplePattern">triple pattern</a>,
	<p>SPARQL also defines an <dfn data-lt="annotation-syntax">annotation syntax</dfn>
	to represent <a href="#defn_TriplePattern">triple patterns</a> that simultaneously
	match reified and asserted triples,

[Tpt]

I got a bit confused by this sentence because it puts at the same level a RDF term and the pattern for a triple that is not a term.

What about something like

Suggested change

	we can use a <a data-cite="RDF12-CONCEPTS#dfn-triple-term">triple term</a>
	or another <a href="#defn_TriplePattern">triple pattern</a>.
	we can use a triple term pattern.

and then define what this "triple term pattern" is.

Also, imho no need to mention plain "triple term"s, they can be seen as a special case of triple term patterns.

[TallTed]

"Special cases" are easily glossed over or misunderstood by new readers. Therefore, to my mind, such "special cases" should almost always be explicitly stated, if they're intended to be understood by such new readers. So, if we accept the edit above, and then 'define what this "triple term pattern" is', that definition should include explicit mention of the special case.