docs: improve template's comment

Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Author: NiccoloFei

templates/metadata.hcl.tmpl

@@ -29,7 +29,11 @@ metadata = {
 
   # TODO: Remove this comment block after customizing the file.
   # `postgresql_parameters`: custom PostgreSQL configuration parameters to be set
-  # for this extension. Usually empty.
+  # for this extension. This is a map of key-value pairs.
+  # This option should be reserved for parameters that are strictly necessary
+  # for the extension to function properly. Avoid setting parameters that just
+  # customize the behavior of the extension.
+  # Usually empty.
   # Used in tests.
   # Example: { "pgaudit.log_client" = "on" }.
   postgresql_parameters    = {}