Intro for GUC overview (#4301)

* add namespace and example

* Apply suggestion from @billy-the-fish

Co-authored-by: Iain Cox <[email protected]>
Signed-off-by: Philip Krauss <[email protected]>

---------

Signed-off-by: Philip Krauss <[email protected]>
Co-authored-by: Iain Cox <[email protected]>

Author: philkra

api/configuration/gucs.md

@@ -10,4 +10,11 @@ import TsdbGucsList from "versionContent/_partials/_timescaledb-gucs.mdx";
 
 You use the following Grand Unified Configuration (GUC) parameters to optimize the behavior of your $SERVICE_LONG.
 
+The namespace of each GUC is `timescaledb`. 
+To set a GUC you specify `<namespace>.<GUC name>`. For example:
+
+```sql
+SET timescaledb.enable_tiered_reads = true;
+```
+
 <TsdbGucsList />