wrote a bit of text about QAST::Want.

docs/qast.markdown

@@ -351,5 +351,19 @@ bounded serialization and a World that builds up a model of the declarative
 aspects of a program.
 
 ## QAST::Want
+QAST::Want nodes will appear in the QAST tree whenever you emit a value, but
+don't yet know in what context it will be used, like 123.
+
+Each sub-node of the Want provides a value for one context out of int, str,
+num, object and void. When a context is known, the last of the matching
+sub-nodes will be used, or the first one if none match. In the case of 123,
+it may be a boxed integer or a native integer value
+
+This notion of context is very code-generation centric, so a want-value of 123
+would not create a sub-node for string context, or else
+
+    my str $x = 123;
+
+would work without complaining.
 
 ## QAST::VM