Add 'returns record' to unsupported functions

Fixes #333

Author: benjie

src/pages/postgraphile/function-restrictions.md

@@ -4,10 +4,17 @@ path: /postgraphile/function-restrictions/
 title: Database Function Restrictions
 ---
 
-PostGraphile supports a wide range of PostgreSQL functions, the only major class
-of functions we don't support currently are VARIADIC functions. We also do not
-support overloaded functions because it's not currently possible to expose them
-neatly over GraphQL.
+PostGraphile supports a wide range of PostgreSQL functions; however we do not
+support:
+
+- VARIADIC functions
+- overloaded functions (because it's not currently possible to expose them
+  neatly over GraphQL)
+- functions that return `record` without any more type information (because we
+  don't know what columns that `record` will contain, and thus cannot convert it
+  to GraphQL)
+  - to solve this, change `record` to be the name of a composite type that
+    you've defined with `CREATE TYPE` (or similar)
 
 ### Thanks