Add missing make_triangle function help. Fix #16406

Author: lbartoletti

resources/function_help/json/make_triangle

@@ -0,0 +1,13 @@
+{
+  "name": "make_triangle",
+  "type": "function",
+  "description": "Creates a triangle polygon.",
+  "variableLenArguments": false,
+  "arguments": [
+	{"arg":"point 1", "description": "first point of the triangle"},
+	{"arg":"point 2", "description": "second point of the triangle"},
+	{"arg":"point 3", "description": "third point of the triangle"}],
+  "examples": [ { "expression":"geom_to_wkt(make_triangle(make_point(0,0), make_point(5,5), make_point(0,10)))", "returns":"'Triangle ((0 0, 5 5, 0 10, 0 0))'"},
+    { "expression":"geom_to_wkt(boundary(make_triangle(make_point(0,0), make_point(5,5), make_point(0,10))))", "returns":"'LineString (0 0, 5 5, 0 10, 0 0)'"}
+    ]
+}