fix doc comment (pgcentralfoundation#2122)

Author: eeeebbbbrrrr

pgrx-macros/src/lib.rs

@@ -492,6 +492,7 @@ Optionally accepts the following attributes:
 * `stable`: Corresponds to [`STABLE`](https://www.postgresql.org/docs/current/sql-createfunction.html).
 * `volatile`: Corresponds to [`VOLATILE`](https://www.postgresql.org/docs/current/sql-createfunction.html).
 * `raw`: Corresponds to [`RAW`](https://www.postgresql.org/docs/current/sql-createfunction.html).
+* `support`: Corresponds to [`SUPPORT`](https://www.postgresql.org/docs/current/sql-createfunction.html) and is the Rust path to a function to act as the SUPPORT function
 * `security_definer`: Corresponds to [`SECURITY DEFINER`](https://www.postgresql.org/docs/current/sql-createfunction.html)
 * `security_invoker`: Corresponds to [`SECURITY INVOKER`](https://www.postgresql.org/docs/current/sql-createfunction.html)
 * `parallel_safe`: Corresponds to [`PARALLEL SAFE`](https://www.postgresql.org/docs/current/sql-createfunction.html).