[tracker/wip/carlosg/serialize-api: 19/19] docs: Add documentation blurb about DESCRIBE resultset format

[Carlos Garnacho <carlosg src gnome org>]

commit 485a00ce956977e20c4baeb6bd9bf6ba736bb1d0
Author: Carlos Garnacho <carlosg gnome org>
Date:   Sun Nov 28 14:26:55 2021 +0100

    docs: Add documentation blurb about DESCRIBE resultset format
    
    It is unclear if it is a deviation or not from the spec, but anyhow it's
    good to spell the decision out.

 docs/reference/libtracker-sparql/sparql-and-tracker.md | 13 +++++++++++++
 1 file changed, 13 insertions(+)
---
diff --git a/docs/reference/libtracker-sparql/sparql-and-tracker.md 
b/docs/reference/libtracker-sparql/sparql-and-tracker.md
index 859985012..da0e184bc 100644
--- a/docs/reference/libtracker-sparql/sparql-and-tracker.md
+++ b/docs/reference/libtracker-sparql/sparql-and-tracker.md
@@ -258,3 +258,16 @@ this full-text search mechanism, by toggling the
 `tracker:fulltextIndexed` property on in the
 text `rdf:Property` instances. See the documentation
 on [defining ontologies](ontologies.html).
+
+## DESCRIBE queries
+
+The [SPARQL documentation](https://www.w3.org/TR/sparql11-query/#describe)
+says:
+
+```
+The DESCRIBE form returns a single result RDF graph containing RDF data about resources.
+```
+
+In order to allow serialization to RDF formats that allow expressing graph information
+(e.g. Trig), DESCRIBE resultsets have 4 columns for subject / predicate / object / graph
+information.