docs(changeset): The Grafast step class 'execute' and 'stream' method…

…s now have a new additional first argument `count` which indicates how many results they must return. This means we don't need to rely on the `values[0].length` trick to determine how many results to return, and thus we can pass through an empty tuple to steps that have no dependencies.

.changeset/sweet-jeans-brake.md

@@ -0,0 +1,12 @@
+---
+"postgraphile": patch
+"@dataplan/json": patch
+"@dataplan/pg": patch
+"grafast": patch
+---
+
+The Grafast step class 'execute' and 'stream' methods now have a new additional
+first argument `count` which indicates how many results they must return. This
+means we don't need to rely on the `values[0].length` trick to determine how
+many results to return, and thus we can pass through an empty tuple to steps
+that have no dependencies.